iommu/crash: Interrupt remapping is also disabled on crash

iommu/crash: Interrupt remapping is also disabled on crash

This fixes a regression side-effect caused by:
  IOMMU: properly check whether interrupt remapping is enabled
    git: fae0372140befb88d890a30704a8ec058c902af8
     hg: 26742:e1ec14bad0cb

On the crash path in nmi_shootdown_cpus(), we shut down the IOMMU, then
disable the IOAPIC.

On systems which support interrupt remapping, the variable iommu_intremap
remains set, meaning that disable_IO_APIC() issues interrupt remapping
invalidate requests.

IOAPIC interrupt remapping used to be conditional on iommu_enabled, but is now
conditional on iommu_intremap, following the above changeset.

This behaviour can be fixed by also indicating that interrupt remapping is not
enabled after shutting down the IOMMU.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>