xenserver: Restore original InterfaceReconfigure*.py on uninstall.

xenserver: Restore original InterfaceReconfigure*.py on uninstall.

The %post script fragment in the RPM spec was moving aside the original
InterfaceReconfigure{,Bridge,Vswitch}.py scripts, but the %postun was
not restoring them.  This commit restores them in %postun.

Without this change, installing the openvswitch RPM on a stock XenServer
and then uninstalling it breaks XenServer networking.

Bug #2624.
Reported-by: Ram Jothikumar <rjothikumar@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>