iommu, x86: Properly handle posted interrupts for IOMMU hotplug

Return error when inserting a new IOMMU which doesn't support posted
interrupts if posted interrupts are already enabled.

Signed-off-by: Feng Wu <feng.wu@intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Joerg Roedel <joro@8bytes.org>
Cc: jiang.liu@linux.intel.com
Cc: iommu@lists.linux-foundation.org
Cc: dwmw2@infradead.org
Link: http://lkml.kernel.org/r/1433827237-3382-11-git-send-email-feng.wu@intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>