Make vfio_migration_realize() adhere to the convention of other realize()
callbacks(like qdev_realize) by returning bool instead of int.

Suggested-by: Cédric Le Goater <clg@redhat.com>
Suggested-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>