virtio-mem-pci: Allow setting nvectors, so we can use MSI-X

virtio-mem-pci: Allow setting nvectors, so we can use MSI-X

Let's do it similar as virtio-balloon-pci. With this change, we can
use virtio-mem-pci on s390x, although plugging will still fail until
properly wired up in the machine.

No need to worry about transitional/non_transitional devices, because they
don't exist for virtio-mem.

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250128185705.1609038-2-david@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>