block: protect path_has_protocol from filenames with colons

path_has_protocol will erroneously return "true" if the colon is part
of a filename.  These names are common with stable device names produced
by udev.  We cannot fully protect against this in case the filename
does not have a path component (e.g. if the current directory is
/dev/disk/by-path), but in the common case there will be a slash before
and path_has_protocol can easily detect that and return false.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -198,14 +198,19 @@ static void bdrv_io_limits_intercept(BlockDriverState *bs,
 /* check if the path starts with "<protocol>:" */
 static int path_has_protocol(const char *path)
 {
+    const char *p;
+
 #ifdef _WIN32
     if (is_windows_drive(path) ||
         is_windows_drive_prefix(path)) {
         return 0;
     }
+    p = path + strcspn(path, ":/\\");
+#else
+    p = path + strcspn(path, ":/");
 #endif
 
-    return strchr(path, ':') != NULL;
+    return *p == ':';
 }
 
 int path_is_absolute(const char *path)