cpu: Move check for NULL CPU model inside the driver

cpu: Move check for NULL CPU model inside the driver

While the check is appropriate for eg. the x86 and generic drivers,
there are some valid ppc64 guest configurations where the CPU
model is supposed to be NULL.

Moving this check from the generic code to the drivers makes it
possible to accomodate both use cases.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1251927