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 <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>