Prepare local artifacts

Most hypervisors cannot boot .qcow2 directly, so guest disks have to be converted. The hypervisor-agnostic prepare command picks a downloaded image, converts it to a guest disk with qemu-img and packs a NoCloud seed into a CIDATA-labelled ISO with the first available of genisoimage, xorriso or mkisofs: user-data/meta-data built from the [cloudinit] settings plus a DHCP network-config (needed because Debian cloud images do not fall back to a generated network configuration). Both artifacts are written next to the source image and only differ in suffix:

# debian-13.qcow2 -> debian-13.vdi + debian-13.iso
qm-template prepare debian-13

# other hypervisors: vmdk (VMware/VirtualBox), raw or vhdx (Hyper-V)
qm-template prepare --format vmdk debian-13

# override the hostname recorded in the seed
qm-template prepare --vm-name debian-13-vbox debian-13

# preview every command without running or writing anything
qm-template prepare --dry-run debian-13

Supported formats are vdi (default), vmdk, qcow2, raw and vhdx; the extension follows the format. Choosing qcow2 for a .qcow2 source is rejected because it would overwrite the source image. For VirtualBox, attach the .vdi as a SATA hard disk and the seed ISO as a CD-ROM. Use the generic/genericcloud image variants: Debian's nocloud variant does not run Cloud-Init.

Like create, prepare requires a login method: at least one SSH key or a password. With only a password, the seed enables password login. An existing guest disk is kept and only the seed ISO is rebuilt, since converting is expensive and the seed derives from the [cloudinit] configuration; pass --force/-f to convert again.