labs: fix libguestfs-tools installation

Supply an answer for the debconf prompt during libguestfs-tools
installation. The current lack of such an answer results in warnings
about "Use of uninitialized value" or a hanging script.

Change-Id: I23888737f24c5f4cd3c8e88ebba0701be85c1d87
Merge "labs: fix libguestfs-tools installation"