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

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

Most virtio-pci devices allow MSI-X. Add it to virtio-balloon-pci, but
only enable it in new machine types, so we don't break migration of
existing machine types between different qemu versions.

This copies what was done for virtio-rng-pci in:
9ea02e8f1306 ("virtio-rng-pci: Allow setting nvectors, so we can use MSI-X")
bad9c5a5166f ("virtio-rng-pci: fix migration compat for vectors")
62bdb8871512 ("virtio-rng-pci: fix transitional migration compat for vectors")

Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Reza Arbab <arbab@linux.ibm.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Message-ID: <20250115161425.246348-1-arbab@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>