vtd: avoid redundant context mapping

vtd: avoid redundant context mapping

After changeset 18934 (VT-d: Fix PCI-X device assignment), my assigned
PCI E1000 NIC doesn't work in guest.

The NIC is 03:00.0. Its parent bridge is: 00:1e.0.
In domain_context_mapping():
   case DEV_TYPE_PCI:
   After we domain_context_mapping_one() 03:00.0 and 00:1e.0, the
   'secbus' is 3 and 'bus' is 0,  so we domain_context_mapping_one()
   03:00.0 again -- this redundant invocation returns -EINVAL because
   we have created the mapping but haven't changed pdev->domain from
   Dom0 to a new domain at this time and eventually the
   XEN_DOMCTL_assign_device hypercall returns a failure.

The attached patch detects this case and avoids the redundant
invocation.

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