virtio_pci: introduce vector allocation fallback for slow path virtqueues

If there is not enough vectors to satisfy all virtqueues, currently
the fallback is to use one vector for all virtqueues.

That may be unnecessary in some cases, when there is enough vectors per
data queues.

Introduce another fallback policy that tries to allocate vector for all
data queues, however for slow path queues (control/admin) it shares
config vector.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Message-Id: <20240716113552.80599-6-jiri@resnulli.us>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>