xenserver: Merge upstream changes and drop pre-5.6.100 support.

Citrix have made modifications to the various interface configuration scripts
that OVS replaces, so at present those modifications are lost when OVS is
installed on e.g. XenServer 5.6.100. This commit applies those changes while
preserving OVS-specific modifications that have been made in the interim.

One major change introduced by these updates is the removal of support for
XenServer releases prior to 5.6.100.

This commit also updates the OVS-supplied xen-bugtool in similar fashion. We
will soon remove xen-bugtool from OVS altogether and move the added OVS
functionality to an XML extension as described in the Supplemental Pack DDK
guide.