KVM: enable PCI multiple-segments for pass-through device

Enable optional parameter (default 0) - PCI segment (or domain) besides
BDF, when assigning PCI device to guest.

Signed-off-by: Zhai Edwin <edwin.zhai@intel.com>
Acked-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>