Introduce the qemu_vfio_find_fixed/temp_iova helpers which
respectively allocate IOVAs from the bottom/top parts of the
usable IOVA range, without picking within host IOVA reserved
windows. The allocation remains basic: if the size is too big
for the remaining of the current usable IOVA range, we jump
to the next one, leaving a hole in the address map.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Message-id: 20200929085550.30926-3-eric.auger@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>