vfio: Add load state functions to SaveVMHandlers

vfio: Add load state functions to SaveVMHandlers

Sequence  during _RESUMING device state:
While data for this device is available, repeat below steps:
a. read data_offset from where user application should write data.
b. write data of data_size to migration region from data_offset.
c. write data_size which indicates vendor driver that data is written in
   staging buffer.

For user, data is opaque. User should write data in the same order as
received.

Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
Reviewed-by: Neo Jia <cjia@nvidia.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>