vfio/common: Extract code from vfio_get_dirty_bitmap() to new function

vfio/common: Extract code from vfio_get_dirty_bitmap() to new function

Extract the VFIO_IOMMU_DIRTY_PAGES ioctl code in vfio_get_dirty_bitmap()
to its own function.

This will help the code to be more readable after next patch will add
device dirty page bitmap sync functionality.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20230307125450.62409-12-joao.m.martins@oracle.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>