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/curl.c b/block/curl.c
index 419f7c8..ef5252d 100644
--- a/block/curl.c
+++ b/block/curl.c
@@ -1034,7 +1034,7 @@
 
     .instance_size              = sizeof(BDRVCURLState),
     .bdrv_parse_filename        = curl_parse_filename,
-    .bdrv_file_open             = curl_open,
+    .bdrv_open                  = curl_open,
     .bdrv_close                 = curl_close,
     .bdrv_co_getlength          = curl_co_getlength,
 
@@ -1053,7 +1053,7 @@
 
     .instance_size              = sizeof(BDRVCURLState),
     .bdrv_parse_filename        = curl_parse_filename,
-    .bdrv_file_open             = curl_open,
+    .bdrv_open                  = curl_open,
     .bdrv_close                 = curl_close,
     .bdrv_co_getlength          = curl_co_getlength,
 
@@ -1072,7 +1072,7 @@
 
     .instance_size              = sizeof(BDRVCURLState),
     .bdrv_parse_filename        = curl_parse_filename,
-    .bdrv_file_open             = curl_open,
+    .bdrv_open                  = curl_open,
     .bdrv_close                 = curl_close,
     .bdrv_co_getlength          = curl_co_getlength,
 
@@ -1091,7 +1091,7 @@
 
     .instance_size              = sizeof(BDRVCURLState),
     .bdrv_parse_filename        = curl_parse_filename,
-    .bdrv_file_open             = curl_open,
+    .bdrv_open                  = curl_open,
     .bdrv_close                 = curl_close,
     .bdrv_co_getlength          = curl_co_getlength,