libxl: passthrough: avoid passing through devices not owned by pciback

libxl: passthrough: avoid passing through devices not owned by pciback

This patch makes sure the passthrough device belongs to pciback before
allow them passthrough to the guest.  There are still many other
checks missing.

xm terminates the guest startup process when this type of condition is
found.  This patch just allows the guest to continue to boot but with
no device passthrough.

Signed-off-by: Allen Kay <allen.m.kay@intel.com>
Signed-off-by: Xudong Hao <xudong.hao@intel.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>