vt-d: Support intra-domain shared interrupt.

vt-d: Support intra-domain shared interrupt.

Inter-domain shared interrupt has been supported by timeout method,
but it still doesn't support intra-domain shared interrupt, that is
assigning multiple devices which share a physical irq to the same
domain. This patch implements intra-domain shared interrupt
support. In addition, this patch maps link to guest device/intx
instead of directly mapping isairq in pt_irq_create_bind_vtd(),
because at this point the isairqs got from pci_link are always 0.

Note that assigning multiple devices to guests which uses PIC to
handle interrupts may be failed, because different links possibly
connect to same irq.

Signed-off-by: Weidong Han <weidong.han@intel.com>