libxl: Extend libxl__ao_device with a libxl__ev_child member

libxl: Extend libxl__ao_device with a libxl__ev_child member

This can be used to fork children to allow the asynchronous execution
of system calls which only come in a synchronous variant. This will
be useful for Remus, in the following patches.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Signed-off-by: Shriram Rajagopalan <rshriram@cs.ubc.ca>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>