OvmfPkg/VirtioFsDxe: implement the wrapper function for FUSE_LOOKUP

Add the VirtioFsFuseLookup() function, for sending the FUSE_LOOKUP 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: <20201216211125.19496-23-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>

OvmfPkg/Include/IndustryStandard/VirtioFs.h

@@ -81,6 +81,11 @@ typedef struct {
 //
 #define VIRTIO_FS_FUSE_ROOT_DIR_NODE_ID 1
 
+//
+// Distinguished errno values.
+//
+#define VIRTIO_FS_FUSE_ERRNO_ENOENT (-2)
+
 //
 // File mode bitmasks.
 //
@@ -107,6 +112,7 @@ typedef struct {
 // FUSE operation codes.
 //
 typedef enum {
+  VirtioFsFuseOpLookup      =  1,
   VirtioFsFuseOpForget      =  2,
   VirtioFsFuseOpMkDir       =  9,
   VirtioFsFuseOpOpen        = 14,