block: do not abuse EMEDIUMTYPE

Returning "Wrong medium type" for an image that does not have a valid
header is a bit weird.  Improve the error by mentioning what format
was trying to open it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/vpc.c

@@ -190,7 +190,8 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
             goto fail;
         }
         if (strncmp(footer->creator, "conectix", 8)) {
-            ret = -EMEDIUMTYPE;
+            error_setg(errp, "invalid VPC image");
+            ret = -EINVAL;
             goto fail;
         }
         disk_type = VHD_FIXED;