xenserver: Use kernel uname version for XenServer 6.5

xenserver: Use kernel uname version for XenServer 6.5

In XenServer 6.5, multiple kernel packages with different
rpm versions can have the same uname.  So, it is not
necessary for openvswitch kernel module to require the
exact rpm version.  Instead, the kernel module package
should check the uname version.

This commit will add a new variable %{kernel_uname} to
specify whether to use kernel uname version or kernel
rpm version as requirement.

When %{kernel_name} is used, openvswitch-module will have
"Requires: kernel-uname-r = <uname version>" set instead of
"Requires: kernel = <version>".

Reported-by: Gosen Chien <astgosen@ccu.edu.tw>
Signed-off-by: Edwin Chiu <echiu@vmware.com>
Signed-off-by: Alex Wang <alexw@nicira.com>