问题出现的情况：
一，
使用在高版本kvm安装的虚拟机的配置xml文件，该虚拟机启动的时候报错如下：
[root@centos1 ~]# virsh create centos.xml
error: Failed to create domain from centos.xml
error: internal error: process exited while connecting to monitor: qemu-kvm: -machine pc-i440fx-rhel7.6.0,accel=kvm,usb=off,dump-guest-core=off: Unsupported machine type
Use -machine help to list supported machines!
[root@centos1 ~]# systemctl status libvirtd
● libvirtd.service - Virtualization daemon
Loaded: loaded (/usr/lib/systemd/system/libvirtd.service; enabled; vendor preset: enabled)
Active: active (running) since Mon 2022-05-09 20:41:10 CST; 1min 18s ago
Docs: man:libvirtd(8)
https://libvirt.org
Main PID: 1620 (libvirtd)
Tasks: 19 (limit: 32768)
CGroup: /system.slice/libvirtd.service
├─1286 /usr/sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/libexec/libvirt_leaseshelpe...
├─1287 /usr/sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/libexec/libvirt_leaseshelpe...
└─1620 /usr/sbin/libvirtd
May 09 20:41:16 centos1 libvirtd[1620]: 2022-05-09 12:41:16.846+0000: 1623: info : hostname: centos1
May 09 20:41:16 centos1 libvirtd[1620]: 2022-05-09 12:41:16.846+0000: 1623: error : qemuMonitorOpenUnix:379 : failed to connect to monitor ... process
May 09 20:41:16 centos1 libvirtd[1620]: 2022-05-09 12:41:16.846+0000: 1623: error : qemuProcessReportLogError:1924 : internal error: proces...ine type
May 09 20:41:16 centos1 libvirtd[1620]: Use -machine help to list supported machines!
May 09 20:41:23 centos1 libvirtd[1620]: 2022-05-09 12:41:23.493+0000: 1623: error : qemuMonitorOpenUnix:379 : failed to connect to monitor ... process
May 09 20:41:23 centos1 libvirtd[1620]: 2022-05-09 12:41:23.493+0000: 1623: error : qemuProcessReportLogError:1924 : internal error: proces...ine type
May 09 20:41:23 centos1 libvirtd[1620]: Use -machine help to list supported machines!
May 09 20:42:19 centos1 libvirtd[1620]: 2022-05-09 12:42:19.295+0000: 1623: error : qemuMonitorOpenUnix:379 : failed to connect to monitor ... process
May 09 20:42:19 centos1 libvirtd[1620]: 2022-05-09 12:42:19.295+0000: 1623: error : qemuProcessReportLogError:1924 : internal error: proces...ine type
May 09 20:42:19 centos1 libvirtd[1620]: Use -machine help to list supported machines!
Hint: Some lines were ellipsized, use -l to show in full.二，
两个版本的对比：
安装虚拟机所使用的kvm版本（注意了，这个版本的kvm需要高版本的内核支持，说人话就是要升级内核才可以正常使用kvm）：
[root@centos1 ~]# virsh -c qemu:///system
Welcome to virsh, the virtualization interactive terminal.
Type: 'help' for help with commands
'quit' to quit
virsh # version
Compiled against library: libvirt 4.5.0
Using library: libvirt 4.5.0
Using API: QEMU 4.5.0
Running hypervisor: QEMU 2.12.0上面报错的kvm版本：
[root@centos1 ~]# virsh -c qemu:///system
Welcome to virsh, the virtualization interactive terminal.
Type: 'help' for help with commands
'quit' to quit
virsh # version
Compiled against library: libvirt 4.5.0
Using library: libvirt 4.5.0
Using API: QEMU 4.5.0
Running hypervisor: QEMU 1.5.3xml文件的machine标签内内容：
<domain type='kvm'>
<name>centos</name>
<uuid>a15efb24-5075-46a8-9732-8e489ece5aba</uuid>
<memory unit='KiB'>1048576</memory>
<currentMemory unit='KiB'>1048576</currentMemory>
<vcpu placement='static'>1</vcpu>
<os>
<type arch='x86_64' machine='pc-i440fx-rhel7.6.0'>hvm</type>
<boot dev='hd'/>
?
三，
解决方法：
使用命令/usr/libexec/qemu-kvm -machine help?查看支持的类型
[root@centos1 ~]# /usr/libexec/qemu-kvm -machine help
Supported machines are:
none empty machine
pc RHEL 7.0.0 PC (i440FX + PIIX, 1996) (alias of pc-i440fx-rhel7.0.0)
pc-i440fx-rhel7.0.0 RHEL 7.0.0 PC (i440FX + PIIX, 1996) (default)
rhel6.6.0 RHEL 6.6.0 PC
rhel6.5.0 RHEL 6.5.0 PC
rhel6.4.0 RHEL 6.4.0 PC
rhel6.3.0 RHEL 6.3.0 PC
rhel6.2.0 RHEL 6.2.0 PC
rhel6.1.0 RHEL 6.1.0 PC
rhel6.0.0 RHEL 6.0.0 PC发现里面有个alias of pc-i440fx-rhel7.0.0 ，而原文件内是alias of pc-i440fx-rhel7.6.0， 因此，将6改成0，再次启动虚拟机没有报错，此问题彻底解决。
总结：kvm版本需要注意，选择一个合适的kvm版本很重要。
