qemu-nbd: Use program name in error messages

qemu-nbd: Use program name in error messages

This changes output from:

$ qemu-nbd nosuch
Failed to blk_new_open 'nosuch': Could not open 'nosuch': No such file or directory

to something more consistent with qemu-img and qemu:

$ qemu-nbd nosuch
qemu-nbd: Failed to blk_new_open 'nosuch': Could not open 'nosuch': No such file or directory

Update the lone affected test to match.  (Hmm - is it sad that we don't
do much testing of expected failures?)

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20181215135324.152629-2-eblake@redhat.com>