define qemu process user and group in configure.ac for Ubuntu

Ubuntu qemu process started by libvirt is libvirt-qemu:kvm, and there
is no group named qemu, only group kvm. This patch sets the correct
value of the variable in configure.ac .

The patch is on going work of porting VDSM to Ubuntu
http://www.ovirt.org/VDSM_on_Ubuntu

Change-Id: I78392b4edcf243e8a2d65b8b18436141e3ed86ea
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/12915
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>