net/virtio: force IOVA as VA mode for virtio-user

net/virtio: force IOVA as VA mode for virtio-user

At least Vhost-user backend of Virtio-user PMD requires
IOVA as VA mode. Until now, it was implemented as a hack
by forcing to use mbuf's buf_addr field instead of buf_iova.

This patch removes all this logic and just fails probing
if IOVA as VA mode is not selected. It simplifies the
code overall, and removes some bus-specific logic from
generic virtio_ethdev.c.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>