SW: add qemu-guest-agent to GRML_FULL

With qemu-guest-agent service being present we support retrieving
network information from a VM, as supported with e.g. Proxmox
v5.2 (verified with qemu-guest-agent v1:2.8+dfsg-6 on grml64-full
2017.05).

Adding only to GRML_FULL as on GRML_SMALL it would pull in
libglib2.0-0 and add ~6MB of disk space, while on GRML_FULL
it's only ~1MB total.