vhost: flush IOTLB cache on new mem table handling

vhost: flush IOTLB cache on new mem table handling

IOTLB entries contain the host virtual address of the guest
pages. When receiving a new VHOST_USER_SET_MEM_TABLE request,
the previous regions get unmapped, so the IOTLB entries, if any,
will be invalid. It does cause the vhost-user process to
segfault.

This patch introduces a new function to flush the IOTLB cache,
and call it as soon as the backend handles a VHOST_USER_SET_MEM
request.

Fixes: 69c90e98f483 ("vhost: enable IOMMU support")
Cc: stable@dpdk.org
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Jens Freimann <jfreimann@redhat.com>