xenserver: Only replace XenServer hosts scripts that we recognize.

xenserver: Only replace XenServer hosts scripts that we recognize.

Open vSwitch comes with slightly improved versions of upstream Citrix
XenSever host scripts.  Until now, the Open vSwitch RPMs have always
installed its own versions of the scripts over the Citrix versions.  This
commit changes behavior to replace the scripts only if we recognize the
exact versions being replaced.  This approach seems safer given that the
benefits of the OVS changes are minor and we don't want to break any
upstream changes inadvertently.

Signed-off-by: Ben Pfaff <blp@nicira.com>