vtd: interrupt remapping: be more defensive

vtd: interrupt remapping: be more defensive

1) A buggy BIOS may not report IOAPIC in DRHD. Currently we still try
to enable IR while the IOAPIC RTEs are still in non-remappable format
and the host would hang. The patch detects this case and will not try
to enable IR.

2) Currently HPET's MSI mode doesn't work if IR is enabled because we
have no code to allocate IRTE for it. Luckily this HW configuration is
rather rarely at present, we can just work it around by only using
HPET's IOAPIC mode for now.

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