Clear stolen linkage on device removal

Clear stolen linkage on device removal

Commit 04c391d9 "Replace copying network settings with identity stealing" to
XENVIF changed the way that network settings are preserved when replacing
an emulated NIC with a PV network device. This change means that both the
emulated NIC and PV device have the same binding to the Windows network
stack. Thus, if we do not destroy that binding prior to uninstallation of
the PV network driver, the stack will be torn down by the network class
uninstall code rather than left in place for the emulated NIC to use after
reboot.

This patch hence adds code to the XENNET co-installer to remove stolen
linkage information from the registry in the pre-remove phase.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>