vhost-user-scsi: prevent using uninitialized vqs

Of the 3 virtqueues, seabios only sets cmd, leaving ctrl
and event without a physical address. This can cause
vhost_verify_ring_part_mapping to return ENOMEM, causing
the following logs:

qemu-system-x86_64: Unable to map available ring for ring 0
qemu-system-x86_64: Verify ring failure on region 0

The qemu commit e6cc11d6
has already resolved the issue for vhost scsi devices but
the fix was never applied to vhost-user scsi devices.

Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1560299717-177734-1-git-send-email-raphael.norwitz@nutanix.com
Message-Id: <1560299717-177734-1-git-send-email-raphael.norwitz@nutanix.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>