cpu: Run arch specific code for virCPUUpdate for all custom CPUs

cpu: Run arch specific code for virCPUUpdate for all custom CPUs

Until now, the function returned immediately when the guest CPU
definition did not use optional features or minimum match. Clearly,
there's nothing to be updated according to the host CPU in this case,
but the arch specific code may still want to do some compatibility
updates based on the model and features used in the guest CPU
definition.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Tim Wiederhake <twiederh@redhat.com>