block: fix BDRV_O_SNAPSHOT protocol detection

realpath(3) is used to get an absolute path to the image file when
creating a -drive snapshot=on temporary qcow2.  This does not work for
protocols since their filenames ("proto:foo:...") do not correspond to
file system paths.

Commit 7c96d46e ("Let snapshot work with
protocols") skipped realpath(3) for protocols.  Later on the "raw"
format was introduced and broke the check.

Use path_has_protocol(filename) to decide if this image uses a protocol
or a filename.
Reported-by: NRichard Jones <rjones@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -830,7 +830,6 @@ int bdrv_open(BlockDriverState *bs, const char *filename, QDict *options,
     if (flags & BDRV_O_SNAPSHOT) {
         BlockDriverState *bs1;
         int64_t total_size;
-        int is_protocol = 0;
         BlockDriver *bdrv_qcow2;
         QEMUOptionParameter *options;
         char backing_filename[PATH_MAX];
@@ -847,9 +846,6 @@ int bdrv_open(BlockDriverState *bs, const char *filename, QDict *options,
         }
         total_size = bdrv_getlength(bs1) & BDRV_SECTOR_MASK;
 
-        if (bs1->drv && bs1->drv->protocol_name)
-            is_protocol = 1;
-
         bdrv_delete(bs1);
 
         ret = get_tmp_filename(tmp_filename, sizeof(tmp_filename));
@@ -858,7 +854,7 @@ int bdrv_open(BlockDriverState *bs, const char *filename, QDict *options,
         }
 
         /* Real path is meaningless for protocols */
-        if (is_protocol) {
+        if (path_has_protocol(filename)) {
             snprintf(backing_filename, sizeof(backing_filename),
                      "%s", filename);
         } else if (!realpath(filename, backing_filename)) {