conf: Move VFIO AP validation from post parse to QEMU validation code

conf: Move VFIO AP validation from post parse to QEMU validation code

VFIO AP has a limitation on a single device per domain, however, when
commit 11708641 added the support for vfio-ap, check for this limitation
was performed as part of the post parse code. Generally, checks like that
should be performed within the driver's validation callback to eliminate
any slight chance of failing in post parse, which could potentially
result in the domain XML config vanishing.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>