Packaging: make libvirtd env file path and qemu process user group configurable

In Ubuntu, qemu user is libvirt-qemu, qemu group is kvm. libvirt daemon
environment file is /etc/default/libvirt-bin. This patch make these
configurable by adding the following options.

--with-libvirt-service-default=/etc/default/libvirt-bin
--with-qemu-user=libvirt-qemu
--with-qemu-group=kvm

The default value are based on RH family so Fedora and RHEL can safely
ignore these options.

Change-Id: I11830b95257655bdc1fe2ba54861655fdcdb861e
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/19007
Reviewed-by: Alon Bar-Lev <alonbl@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>