assistant: keep prepared iso bootable on uefi with flash drives

assistant: keep prepared iso bootable on uefi with flash drives

By mapping files into the ISO, the UUID for the partitions change as
they depend on the timestamp. The result is, that grub cannot find its
partition anymore and the user ends up on the grub shell.

This only happens when booting from a blockdev in UEFI mode. E.g. a USB
flash drive. Alternatively one can `dd` the ISO to a small (2GiB) VM
disk and mark it as the first boot device.

When booting in legacy mode or via CDROM (e.g. pass through via IPMI),
it worked.

Xorriso can report the commands needed to recreate the source ISO. The
'-volume_date uuid' is the one needed to override the same UUIDs. We
therefore read it first from the source iso and pass it as parameter
whenever we inject a file into the iso.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>
Reviewed-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Tested-by: Stoiko Ivanov <s.ivanov@proxmox.com>