qemu_vm: Making qemu creation module honor cpu_model unconditionally

When we introduced qemu CPU model configuration, we opted
for robustness, so we wouldn't add a cpu_model to the
command line in case it was not showing on qemu -cpu help
text. This is bad because:

1) It was all done very silently
2) This makes negative cpu model testing unfeasible

So let's honor the setting, and print a error message,
so people have a better idea of what's going on. In
such cases, the bad param will be passed to qemu, it
will fail and a VMCreateError will be thrown.

CC: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>