libxl: disallow PCI device assignment for HVM guest when PoD is enabled

libxl: disallow PCI device assignment for HVM guest when PoD is enabled

This replicates a Xend behavior, see ec789523749 ("xend: Dis-allow
device assignment if PoD is enabled.").

This change is restricted to HVM guest, as only HVM is relevant in the
counterpart in Xend. We're late in release cycle so the change should
only do what's necessary. Probably we can revisit it if we need to do
the same thing for PV guest in the future.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>