vfio: Change default dirty pages tracking behavior during migration

vfio: Change default dirty pages tracking behavior during migration

By default dirty pages tracking is enabled during iterative phase
(pre-copy phase).
Added per device opt-out option 'x-pre-copy-dirty-page-tracking' to
disable dirty pages tracking during iterative phase. If the option
'x-pre-copy-dirty-page-tracking=off' is set for any VFIO device, dirty
pages tracking during iterative phase will be disabled.

Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>