CA-136104: Adding better kernel version detection: http://lists.xen.org/archives/html/xen-devel/2013-12/msg01185.html

This prevents the failing on the call to io_setup with EINVAL. Tapdisk
currently detects kernel version for eventfd by looking for 'x.y.z',
which fails on the newer kernel versions.

Credit goes to James Harper from
http://lists.xen.org/archives/html/xen-devel/2013-12/msg01185.html

Signed-off-by: Antony Messerli <antony@mes.ser.li>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #79 on xapi-project/blktap
(cherry picked from commit 74152b0374041d79b1a25a5ac4f74f0daee1398a)