virt: stats: move exited vm stats in a method

this patch moves the ExitedVmStats handling in
a separate method. No functional changes.

Change-Id: I7492086bfc7ce02b6f7035fef8d04fbc0b7fbc43
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26547
Reviewed-by: Dan Kenigsberg <danken@redhat.com>