vhost-user-blk: prevent using uninitialized vqs

vhost-user-blk: prevent using uninitialized vqs

Same rational as: e6cc11d64fc998c11a4dfcde8fda3fc33a74d844

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

This has already been fixed for vhost scsi devices and was
recently vhost-user scsi devices. This commit fixes it for
vhost-user-blk devices.

Suggested-by: Phillippe Mathieu-Daude <[email protected]>
Signed-off-by: Raphael Norwitz <[email protected]>
Message-Id: <1566498865[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>