block: Make bdrv_create_file() ':' handling consistent

Filenames may start with "<protocol>:" to explicitly use a protocol like
nbd.  Filenames with unknown protocols are rejected in most of QEMU
except for bdrv_create_file().  Even if a file with an invalid filename
can be created, QEMU cannot use it since all the other relevant
functions reject such paths.  Make bdrv_create_file() consistent.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -215,7 +215,7 @@ int bdrv_create_file(const char* filename, QEMUOptionParameter *options)
 
     drv = bdrv_find_protocol(filename);
     if (drv == NULL) {
-        drv = bdrv_find_format("file");
+        return -ENOENT;
     }
 
     return bdrv_create(drv, filename, options);