RHEL-9 base images are whole-disk images with the /boot/efi partition
correctly set up for EFI Secure Boot. This doesn't work with
extract-image because it only mounts the root partition, leaving
/boot/efi empty even though grub2-efi & shim packages are "installed".
This change mounts discovered partitions to mnt/boot, mnt/boot/efi so
all content can be extracted from the image.
Partition detection is done by reading block device attributes and
matching on Boot Loader Specification[1] UIDs or labels as observed in
supported base images.
[1] https://systemd.io/BOOT_LOADER_SPECIFICATION/
Change-Id: I8487002a18ae6ca98609ab68d92ae9173a2b864f
0630b3cb69