xenserver: Clean up /usr/sbin/brctl dangling symlink.

xenserver: Clean up /usr/sbin/brctl dangling symlink.

Commit 54f16a10 removed Open vSwitch's /usr/sbin/brctl replacement for
XenServer, but because the RPM doesn't restore the original files on upgrade,
an upgrade leaves /usr/sbin/brctl a broken symlink to the removed
/usr/share/openvswitch/scripts/brctl.

This commit adds a one-time cleanup to restore the original XenServer-supplied
/usr/sbin/brctl. It also changes the RPM to restore all the replaced files with
their XenServer originals at post-uninstall time during an upgrade transaction,
just as uninstall has done up to now. This should prevent the same problem in
the future.

Bug #4667.