For DMA in native system, operating system depends on platform
flushes icache of memory touched by DMA operations.
But as to virtual DMA of virtual machine, dma emulation code has to
use explicit instructions to flush icahce,otherwise,
guest may use old icache and leads to guest's crash.

Signed-off-by: Xiantao Zhang <xiantao.zhang@intel.com>
Signed-off-by: Yang Zhang <yang.zhang@intel.com>