Documentation: Add note about dpdkvhostuser and IOMMU.

The docs describe IOMMU support for dpdkvhostuserclient ports,
but it is not mentioned in the section about dpdkvhostuser
ports. Add an explicit note to say IOMMU is not supported for
dpdkvhostuser ports.

CC: Maxime Coquelin <maxime.coquelin@redhat.com>
Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>

Documentation/topics/dpdk/vhost-user.rst

@@ -105,6 +105,9 @@ Once the vhost-user ports have been added to the switch, they must be added to
 the guest. There are two ways to do this: using QEMU directly, or using
 libvirt.
 
+.. note::
+   IOMMU is not supported with vhost-user ports.
+
 Adding vhost-user ports to the guest (QEMU)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~