[FEATURE REQUEST] Update `amigen9-build.sh` to properly function in Azure

[ferricoxide]

When attempting to use the azure-chroot builder (actually, a modified version thereof), the current logic for computing the installation-target fails. Need to update so that it functions, correctly.

• Logic needs to properly-compute the target disk, whether or not the (chroot) builder-host has a waagent-managed (ephemeral) disk attached
• Logic needs to properly-compute the target disk, whether or not the (chroot) builder-host is using standard/legacy SCSI devices or using NVMe devices