core: Add boot time to the getVdsStats API

Read boot time from /proc/stat and report in host stats

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1070348
Change-Id: I319e619cdaecac2f86d0154e3adbb3beda9c57d6
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25877
Reviewed-by: Dan Kenigsberg <danken@redhat.com>