qemu: Make sure host-model uses CPU model supported by QEMU

qemu: Make sure host-model uses CPU model supported by QEMU

When reconnecting to a running domain started by old libvirt, which did
not change host-model into a custom CPU definition, we replace the CPU
definition with a specific CPU model from host capabilities. However,
that CPU model may not be supported by the running qemu process. We need
to translate the CPU model to one of the models which libvirt could have
used when starting the domain.

https://bugzilla.redhat.com/show_bug.cgi?id=1521202

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>