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: NEric Blake <eblake@redhat.com>
Reviewed-by: NRichard W.M. Jones <rjones@redhat.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20181215135324.152629-2-eblake@redhat.com>

qemu-nbd.c

@@ -571,6 +571,7 @@ int main(int argc, char **argv)
 #endif
 
     module_call_init(MODULE_INIT_TRACE);
+    error_set_progname(argv[0]);
     qcrypto_init(&error_fatal);
 
     module_call_init(MODULE_INIT_QOM);

tests/qemu-iotests/233.out

@@ -27,7 +27,7 @@ virtual size: 64M (67108864 bytes)
 disk size: unavailable
 
 == check TLS with different CA fails ==
-option negotiation failed: Verify failed: No certificate was found.
+qemu-nbd: option negotiation failed: Verify failed: No certificate was found.
 qemu-img: Could not open 'driver=nbd,host=127.0.0.1,port=PORT,tls-creds=tls0': The certificate hasn't got a known issuer
 
 == perform I/O over TLS ==