cpu: Avoid adding <vendor> to custom CPUs

cpu: Avoid adding <vendor> to custom CPUs

Guest CPU definitions with mode='custom' and missing <vendor> are
expected to run on a host CPU from any vendor as long as the required
CPU model can be used as a guest CPU on the host. But even though no CPU
vendor was explicitly requested we would sometimes force it due to a bug
in virCPUUpdate and virCPUTranslate.

The bug would effectively forbid cross vendor migrations even if they
were previously working just fine.

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