vfio/migration: Report only stop-copy size in vfio_state_pending_exact()

vfio/migration: Report only stop-copy size in vfio_state_pending_exact()

vfio_state_pending_exact() is used to update migration core how much
device data is left for the device migration. Currently, the sum of
pre-copy and stop-copy sizes of the VFIO device are reported.

The pre-copy size is obtained via the VFIO_MIG_GET_PRECOPY_INFO ioctl,
which returns the amount of device data available to be transferred
while the device is in the PRE_COPY states.

The stop-copy size is obtained via the VFIO_DEVICE_FEATURE_MIG_DATA_SIZE
ioctl, which returns the total amount of device data left to be
transferred in order to complete the device migration.

According to the above, current implementation is wrong -- it reports
extra overlapping data because pre-copy size is already contained in
stop-copy size. Fix it by reporting only stop-copy size.

Fixes: eda7362af959 ("vfio/migration: Add VFIO migration pre-copy support")
Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>