Set PBS_QEMU_DEBUG=1 on a command that starts a VM and then connect to
the debug root shell via:
minicom -D \unix#/run/proxmox-backup/file-restore-serial-10.sock
or similar.
Note that this requires 'proxmox-backup-restore-image-debug' to work,
the postinst script is updated to also generate the corresponding image.
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>