OvmfPkg/VirtioFsDxe: implement the wrapper function for FUSE_FLUSH

Add the VirtioFsFuseFlush() function, for sending the FUSE_FLUSH command
to the Virtio Filesystem device.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3097
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20201216211125.19496-15-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>

OvmfPkg/Include/IndustryStandard/VirtioFs.h

@@ -88,6 +88,7 @@ typedef enum {
   VirtioFsFuseOpForget      =  2,
   VirtioFsFuseOpRelease     = 18,
   VirtioFsFuseOpFsync       = 20,
+  VirtioFsFuseOpFlush       = 25,
   VirtioFsFuseOpInit        = 26,
   VirtioFsFuseOpOpenDir     = 27,
   VirtioFsFuseOpReleaseDir  = 29,
@@ -141,6 +142,16 @@ typedef struct {
   UINT32 Padding;
 } VIRTIO_FS_FUSE_FSYNC_REQUEST;
 
+//
+// Header for VirtioFsFuseOpFlush.
+//
+typedef struct {
+  UINT64 FileHandle;
+  UINT32 Unused;
+  UINT32 Padding;
+  UINT64 LockOwner;
+} VIRTIO_FS_FUSE_FLUSH_REQUEST;
+
 //
 // Headers for VirtioFsFuseOpInit.
 //