fix #3728: handle machine without type

fix #3728: handle machine without type

libguestfs starts their helper VMs with `-machine accel=..` without a
machine type, and our pve version suffix handling would segfault in that
case. there might be other scripted use cases that are affected as well.

this regression was introduced with the rebase of our patch set on top
of 6.1.0

Fixes: f376b2b9e2fc1f0c8f9c6275e96ede5f423c4126
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>