xenserver: Be compatible with non-XenServer func libraries in init script.

xenserver: Be compatible with non-XenServer func libraries in init script.

XenServer has its function library in /etc/init.d/functions but other
distros have it in different places.  Currently this init script is
specifically intended for XenServer but adding compatibility with other
distros seems like a worthwhile goal.

Also, SuSE does not have the "action" function, so test whether it is
implemented and when not supply a fallback.

Signed-off-by: Patrick Mullaney <pm.mullaney@gmail.com>
[changed to only define action() if not already defined]
Signed-off-by: Ben Pfaff <blp@nicira.com>