add support for setting variables in RPI_EFI.fd

The Raspberry Pi 4 UEFI firmware (RPi4) at https://github.com/pftf/RPi4 ships with a RPI_EFI.fd file that has the CODE+VARS. Please note that this firmware, currently, does not support having a separate VARS file (see #6, #102, #138, #210).

Currently, virt-fw-vars does not seem to support it:

virt-fw-vars -i RPi4_UEFI_Firmware_v1.34/RPI_EFI.fd -p
ERROR: unknown input file format

Would it be possible to support it?