block: Fix protocol detection for Windows devices

We can't assume the file protocol for Windows devices, they need the same
detection as other files for which an explicit protocol is not specified.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -287,16 +287,18 @@ static BlockDriver *find_protocol(const char *filename)
     char protocol[128];
     int len;
     const char *p;
+    int is_drive;
 
     /* TODO Drivers without bdrv_file_open must be specified explicitly */
 
 #ifdef _WIN32
-    if (is_windows_drive(filename) ||
-        is_windows_drive_prefix(filename))
-        return bdrv_find_format("file");
+    is_drive = is_windows_drive(filename) ||
+        is_windows_drive_prefix(filename);
+#else
+    is_drive = 0;
 #endif
     p = strchr(filename, ':');
-    if (!p) {
+    if (!p || is_drive) {
         drv1 = find_hdev_driver(filename);
         if (!drv1) {
             drv1 = bdrv_find_format("file");