vfio/common: Extract vIOMMU code from vfio_sync_dirty_bitmap()

vfio/common: Extract vIOMMU code from vfio_sync_dirty_bitmap()

Extract vIOMMU code from vfio_sync_dirty_bitmap() to a new function and
restructure the code.

This is done in preparation for optimizing vIOMMU device dirty page
tracking. No functional changes intended.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
[ clg: - Rebased on upstream
       - Fixed typo in commit log ]
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>