block: remove separate bdrv_file_open callback

bdrv_file_open and bdrv_open are completely equivalent, they are
never checked except to see which one to invoke.  So merge them
into a single one.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/block.c b/block.c
index dd14ba8..c1cc313 100644
--- a/block.c
+++ b/block.c
@@ -1655,9 +1655,7 @@
     bs->opaque = g_malloc0(drv->instance_size);
 
     assert(!drv->bdrv_needs_filename || bs->filename[0]);
-    if (drv->bdrv_file_open) {
-        ret = drv->bdrv_file_open(bs, options, open_flags, &local_err);
-    } else if (drv->bdrv_open) {
+    if (drv->bdrv_open) {
         ret = drv->bdrv_open(bs, options, open_flags, &local_err);
     } else {
         ret = 0;