xenserver: Make RPM install work again.

xenserver: Make RPM install work again.

The RPM install was generating a database cache in Python pickle format in
/etc/ovs-vswitchd.conf, but interface-reconfigure was looking for it in
XML format in /var/lib/openvswitch/dbcache.  This fixes the problem, by
adding an init-dbcache command to interface-reconfigure and then using that
at RPM install time.

This moves the database cache creation from %pre to %post.  This is
necessary so that interface-reconfigure is available from the install
script.