VT-d: don't reject possibly valid DRHD or RMRR

VT-d: don't reject possibly valid DRHD or RMRR

If a non-zero PCI segment isn't accessible during Xen boot (because
firmware decided to not enter the necessary MMIO space into the E820
table), devices referred to on those segments through DRHD or RMRR
structures should not be rejected just because the devices can't be
found.

This is in line with what is being done in at least one other case
already: Systems with more than one PCI segment (usually high end
ones) are assumed to have valid firmware provided data, while systems
with just segment 0 continue to have their firmware tables validated.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: "Kay, Allen M" <allen.m.kay@intel.com>