Remove the VFIO_DMA_MAP_FLAG_EXEC flag

The VFIO_DMA_MAP_FLAG_EXEC flag has been replaced by the new flag
VFIO_DMA_MAP_FLAG_NOEXEC. Now DMA mappings will not be marked as XN
unless specifically asked for by the user.

Signed-off-by: Antonios Motakis <a.motakis@virtualopensystems.com>