xend: passthrough: add an option pci-passthrough-strict-check

xend: passthrough: add an option pci-passthrough-strict-check

Currently when assigning device to HVM guest, we use the strict check
for HVM guest by default.(For PV guest we use loose check
automatically if necessary.)

When we assign device to HVM guest, if we meet with the co-assignment
issues or the ACS issue (see changeset 20081: 4a517458406f), we could
try changing the option to 'no' -- however, we have to realize this
may incur security issue and we can't make sure the device assignment
could really work properly even after we do this.

The option is located in /etc/xen/xend-config.sxp:
(pci-passthrough-strict-check yes)

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>