Bug#844339: [PATCH v3 3/4] libvirt-daemon-system.postinst: use allocated uid/gid if possible

Use the allocated uid/gid for libvirt-qemu
when creating the user/group, if not taken.

In case it's taken, the user has been asked
to continue or abort the installation, thus
if we are here, it is OK to proceed and not
use the allocated uid/gid.

Signed-off-by: Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>
Signed-off-by: Guido Günther <agx@sigxcpu.org>