While virtio-scsi does support multiqueue, the default number of
interrupt vectors is not enough to actually enable usage of
multiple queues in the driver; this is because with only 2
vectors the driver will not be able to use a separate
interrupt for each request queue.  Derive the desired number
of vectors from the number of request queues.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>