Use partition layout on the iso as isohybrid did

Some legacy firmware are not happy about protective MBR partition table
(even if the boot sector is there). Use isohybrid partition layout -
which presents actual partitions in MBR table "mirrored" from GPT.

Fixes QubesOS/qubes-issues#8462

qubesbuilder/plugins/installer/Makefile

@@ -147,6 +147,7 @@ iso-installer-mkisofs:
 		-eltorito-alt-boot \
 		-e '--interval:appended_partition_2:all::' -no-emul-boot \
 		-graft-points \
+		-part_like_isohybrid \
 		.=$(BASE_DIR)/os \
 		boot/grub2/i386-pc=/usr/lib/grub/i386-pc
 	/usr/bin/implantisomd5 $(BASE_DIR)/iso/$(ISO_NAME).iso