qemu-io qemu-nbd: Use error_report() etc. instead of fprintf()

Just three instances left.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1450452927-8346-16-git-send-email-armbru@redhat.com>
diff --git a/tests/qemu-iotests/131.out b/tests/qemu-iotests/131.out
index 021a04c..ae2412e 100644
--- a/tests/qemu-iotests/131.out
+++ b/tests/qemu-iotests/131.out
@@ -22,7 +22,7 @@
 read 32768/32768 bytes at offset 0
 32 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 == Corrupt image ==
-qemu-io: can't open device TEST_DIR/t.parallels: parallels: Image was not closed correctly; cannot be opened read/write
+can't open device TEST_DIR/t.parallels: parallels: Image was not closed correctly; cannot be opened read/write
 no file open, try 'help open'
 ERROR image was not closed correctly