qemu: Avoid reporting "host" as a supported CPU model

qemu: Avoid reporting "host" as a supported CPU model

"host" CPU model is supported by a special host-passthrough CPU mode and
users is not allowed to specify this model directly with custom mode.
Thus we should not advertise "host" CPU model in domain capabilities.
This worked well on architectures for which libvirt provides a list of
supported CPU models in cpu_map.xml (since "host" is not in the list).
But we need to explicitly filter "host" model out for all other
architectures.

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