OvmfPkg/VirtioFsDxe: implement the wrapper function for FUSE_WRITE

OvmfPkg/VirtioFsDxe: implement the wrapper function for FUSE_WRITE

Add the VirtioFsFuseWrite() function, for sending the FUSE_WRITE command
to the Virtio Filesystem device.

(For avoiding oversized FUSE_WRITE commands, save the maximum write buffer
size that is advertized by the FUSE server, in the session init code.)

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: <20201216211125.19496-39-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>