qemu-img: fix formatting of error message

qemu-img: fix formatting of error message

The error_reportf_err() will not automatically append a
': ' before adding its suffix, so we must include that
in the message we pass it, otherwise we get a badly
formatted message lacking whitespace:

qemu-img: Could not open 'driver=nbd,host=127.0.0.1,port=6666,tls-creds=tls0'Failed to connect socket: Connection refused

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>