xenserver: Fix build spec for XenServer 6.5.

xenserver: Fix build spec for XenServer 6.5.

The latest XenServer 6.5 uses a new way for kernel version naming.
Therein, the kernel flavor could not be found anymore.  Also, the
directory name in 'lib/modules/' becomes a shortened version of
kernel version. e.g.:

[root@localhost ~]# ls /lib/modules/
3.10.0+2

As a workaround, this commit modifies the spec file to make
%{kernel_flavor} optional and %{xen_version} definable by users.
In the long run, I'd like to spend time refining the spec file.

Signed-off-by: Edwin Chiu <echiu@vmware.com>
Signed-off-by: Alex Wang <alexw@nicira.com>