xend: Update info['platform']['pci']

xend: Update info['platform']['pci']

This patch updates info['platform']['pci'] for PCI devices
assignment to domains.

When a domain is started, xend confirms by using xc.test_assign_device
whether PCI devices can be assigned to the domain.  For the
confirmation, info['platform']['pci'] must be an appropriate value.
However, info['platform']['pci'] may be not appropriate.  Because
info['platform']['pci'] isn't almost always updated even if the PCI
device configuration of the domain was changed by using xm
pci-attach/detach.  This patch updates info['platform']['pci'] to the
appropriate value when domains are started.

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>