block: Add special error code for wrong format

The block drivers need a special error code for "wrong format".
From the available error codes EMEDIUMTYPE fits best.
It is not available on all platforms, so a definition in
qemu-common.h and a specific error report are needed.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -617,8 +617,13 @@ DriveInfo *drive_init(QemuOpts *opts, BlockInterfaceType block_default_type)
 
     ret = bdrv_open(dinfo->bdrv, file, bdrv_flags, drv);
     if (ret < 0) {
+        if (ret == -EMEDIUMTYPE) {
+            error_report("could not open disk image %s: not in %s format",
+                         file, drv->format_name);
+        } else {
             error_report("could not open disk image %s: %s",
                          file, strerror(-ret));
+        }
         goto err;
     }
 

include/qemu-common.h

@@ -68,6 +68,9 @@
 #if !defined(ECANCELED)
 #define ECANCELED 4097
 #endif
+#if !defined(EMEDIUMTYPE)
+#define EMEDIUMTYPE 4098
+#endif
 #ifndef TIME_MAX
 #define TIME_MAX LONG_MAX
 #endif