kvm flag is not exposed by default

Hi I found that the kvm flags is not exposed by default, but according to the source code, it should be exposed by default when the CPU Model is a X86CPU.

we have to specifically add "kvm=on" in QEMU custom cpu args like this:
<qemu:arg value='host,kvm=on,+invtsc,+hypervisor'/>

Also the libvirt can't expose kvm because this (libvirt assumes the kvm flag is exposed by default, only "kvm hidden = 'true'" can be used.