qemu: enable initrd support

With 6edcf9dc2e1a ("efi/libstub: EFI_GENERIC_STUB_INITRD_CMDLINE_LOADER
should not default to yes") (included in Linux v5.11) we need to set
EFI_GENERIC_STUB_INITRD_CMDLINE_LOADER=y explicitly in order to enable
initrd support.

Acked-by: Jerome Forissier <jerome@forissier.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>