OvmfPkg/VirtioFsDxe: add a shared wrapper for FUSE_READ / FUSE_READDIRPLUS

Add the VirtioFsFuseReadFileOrDir() function, for sending the FUSE_READ or
FUSE_READDIRPLUS command to the Virtio Filesystem device.

Parsing the structured FUSE_READDIRPLUS output is complex, and cannot be
integrated into the wrapper function. Given that fact, FUSE_READ and
FUSE_READDIRPLUS turn out to need identical low-level handling, except for
the opcode. Hence the shared wrapper function.

(It's prudent to verify whether the FUSE server supports FUSE_READDIRPLUS,
so update the session init code accordingly.)

This is the first FUSE request wrapper function that deals with a variable
size tail buffer.

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: <20201216211125.19496-33-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>

OvmfPkg/VirtioFsDxe/VirtioFsDxe.h

@@ -298,6 +298,17 @@ VirtioFsFuseOpen (
      OUT UINT64    *FuseHandle
   );
 
+EFI_STATUS
+VirtioFsFuseReadFileOrDir (
+  IN OUT VIRTIO_FS *VirtioFs,
+  IN     UINT64    NodeId,
+  IN     UINT64    FuseHandle,
+  IN     BOOLEAN   IsDir,
+  IN     UINT64    Offset,
+  IN OUT UINT32    *Size,
+     OUT VOID      *Data
+  );
+
 EFI_STATUS
 VirtioFsFuseStatFs (
   IN OUT VIRTIO_FS                      *VirtioFs,