net/vhost: add parameter to enable IOMMU feature

Introduce a new iommu-support parameter to Vhost PMD that
passes the RTE_VHOST_USER_IOMMU_SUPPORT flag at vhost
device register time.

Default value is 0, meaning that IOMMU support is disabled
if not specified explicitly.

Example to enable IOMMU support for a given device:

--vdev 'net_vhost0,iface=/tmp/vhost-user2,iommu-support=1'

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Yuanhan Liu <yliu@fridaylinux.org>
Tested-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Acked-by: Mark Kavanagh <mark.b.kavanagh@intel.com>