Reply with UEFI loader when client system architecture is of type 9

As per RFC-4578, PXE clients send their architecture type. For an x86_64 box this can be either 7 (EFI BC / EFI Byte Code, processor agnostic) or 9 (EFI x86-64). Seems like tested Dell boxes (and some IBM ones according to Google) send 7, while my OVMF-based KVM instance sends 9.