virtio-fs: fix MSI-X nvectors calculation

virtio-fs: fix MSI-X nvectors calculation

The following MSI-X vectors are required:
 * VIRTIO Configuration Change
 * hiprio virtqueue
 * requests virtqueues

Fix the calculation to reserve enough MSI-X vectors.  Otherwise guest
drivers fall back to a sub-optional configuration where all virtqueues
share a single vector.

This change does not break live migration compatibility since
vhost-user-fs-pci devices are not migratable yet.

Reported-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20191209110759.35227-1-stefanha@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>