intel_iommu, amd_iommu: allow UNMAP notifiers

x86 vIOMMUs still lack of a complete IOMMU notifier mechanism.
Before that is achieved, let's open a door for vhost DMAR support,
which only requires cache invalidations (UNMAP operations).

Meanwhile, convert hw_error() to error_report() and exit(1), to make
the error messages cleaner and obvious (no CPU registers will be dumped).

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1474606948-14391-4-git-send-email-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
