qemu_validate: Reject virtiofs with bootindex on s390x with CCW

The CCW variant of the 'vhost-user-fs' device in qemu doesn't
deliberately support the 'bootindex' attribute as the machine is unable
to boot from such device.

Reject '<boot order' on non-PCI virtiofs, add tests validating that it's
rejected as well as that virtiofs on PCI-based hosts but without address
specified will be accepted.

Resolves: https://issues.redhat.com/browse/RHEL-22728


Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>