vm-device: Move ExternalDmaMapping trait out of vfio-ioctls

By moving the trait and its VFIO implementation out of vfio-ioctls, we
give anticipate for the move to the vfio-ioctls from rust-vmm.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-device/src/lib.rs

@@ -10,6 +10,7 @@ extern crate vm_memory;
 use std::io;
 
 mod bus;
+pub mod dma_mapping;
 pub mod interrupt;
 
 pub use self::bus::{Bus, BusDevice, Error as BusError};