util: move virStorageFileBackend code into storage_file

It's used only by storage file code so it doesn't make sense to have
it in util directory.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

src/storage_file/storage_source.c

@@ -25,6 +25,7 @@
 #include <unistd.h>
 
 #include "internal.h"
+#include "storage_file_backend.h"
 #include "storage_source.h"
 #include "viralloc.h"
 #include "virerror.h"
@@ -34,7 +35,6 @@
 #include "virlog.h"
 #include "virobject.h"
 #include "virstoragefile.h"
-#include "virstoragefilebackend.h"
 #include "virstoragefileprobe.h"
 #include "virstring.h"
 #include "viruri.h"