OvmfPkg/VirtioFsDxe: erase the dir. entry in EFI_FILE_PROTOCOL.Delete()

OvmfPkg/VirtioFsDxe: erase the dir. entry in EFI_FILE_PROTOCOL.Delete()

At this point, the infrastructure is available for looking up the directly
containing directory of the file in EFI_FILE_PROTOCOL.Delete(), and to
remove the file in that directory by last pathname component. Do so.

The "RM" UEFI shell command will start working only later in the series;
the shell needs more EFI_FILE_PROTOCOL members to function before it calls
Delete().

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: <20201216211125.19496-28-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>