OvmfPkg/VirtioFsDxe: split canon. path into last parent + last component

OvmfPkg/VirtioFsDxe: split canon. path into last parent + last component

Given a canonical pathname (as defined by VirtioFsAppendPath()), different
from "/", introduce a helper function for:

- looking up the NodeId of the most specific parent directory, and

- exposing the last component stand-alone (which is therefore a direct
  child of said parent directory).

This splitting operation will be necessary in multiple subsequent patches.

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: <20201216211125.19496-24-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>