OvmfPkg/VirtioFsDxe: add helper for formatting UEFI basenames

OvmfPkg/VirtioFsDxe: add helper for formatting UEFI basenames

The EFI_FILE_INFO structure, which is output by
EFI_FILE_PROTOCOL.GetInfo(), ends with a flexible CHAR16 array called
"FileName". Add the VirtioFsGetBasename() function, for determining the
required array size, and for filling the array as well.

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: <20201216211125.19496-30-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>