Simplify offline driver installation for xenbus

Drivers installed to an offline Windows image using dism.exe will not
have an ActiveDevice parameter.  Treat a missing ActiveDevice
parameter as meaning "bind to any compatible device", to minimise the
need for registry hacks when using offline driver installation.

Signed-off-by: Michael Brown <mbrown@fensystems.co.uk>
Simplify offline driver installation for xenfilt

Drivers installed to an offline Windows image using dism.exe will not
have Count parameters set for each of the services identified via
Unplug\DISKS and Unplug\NICS.  Treat a missing Count parameter as
meaning "assume that this driver will bind to all compatible devices",
to minimise the need for registry hacks when using offline driver
installation.

Signed-off-by: Michael Brown <mbrown@fensystems.co.uk>