vhost: add flag to enable IOMMU support

vhost: add flag to enable IOMMU support

Qemu versions from v2.7.0 to v2.9.0 have their reply-ack protocol
feature implementation broken with multiqueue. The reply-ack
protocol feature is optional except for IOMMU feature.

This patch introduce a new RTE_VHOST_USER_IOMMU_SUPPORT flag to
enable VIRTIO_F_IOMMU_PLATFORM virtio feature.

By default, the IOMMU support is now disabled.

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>