add iothread/dataplane support

new config option:

iothread: 1|0

This enable iothread/dataplane support, to improve io performance on fast storages

Currently block jobs don't work yet, it's planned for qemu 2.2.
So it's better to not expose yet this option in gui.

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>