qemu: Store host-model CPU in qemu capabilities

qemu: Store host-model CPU in qemu capabilities

Host capabilities provide libvirt's view of the host CPU, but for a
useful support for host-model CPUs we really need a hypervisor's view of
the CPU. And since the view can be differ with emulator, qemu
capabilities is the best place to store the host CPU model.

This patch just copies the CPU model from host capabilities, but this
will change in the future.

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