SECURITY IMPLICATION: this patch fixes a potential race when multiple
threads access the IOMMU IOTLB cache.

Add a per-iommu big lock to protect IOMMU status.  Currently the only
thing to be protected is the IOTLB/context cache, since that can be
accessed even without BQL, e.g., in IO dataplane.

Note that we don't need to protect device page tables since that's fully
controlled by the guest kernel.  However there is still possibility that
malicious drivers will program the device to not obey the rule.  In that
case QEMU can't really do anything useful, instead the guest itself will
be responsible for all uncertainties.

CC: QEMU Stable <qemu-stable@nongnu.org>
Reported-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NPeter Xu <peterx@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 1d9efa73)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>