OvmfPkg/VirtioFsDxe: implement the wrapper function for FUSE_FLUSH

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>