vhost: move cuse only struct to cuse

vhost: move cuse only struct to cuse

vhost cuse is now the last reference of the vhost_device_ctx struct;
move it there, and do a rename to "vhost_cuse_device_ctx", to make
it clear that it's "cuse only".

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Tested-by: Rich Lane <rich.lane@bigswitch.com>
Acked-by: Rich Lane <rich.lane@bigswitch.com>