libxl: Make killing of device model asynchronous

libxl: Make killing of device model asynchronous

Or at least, give it an asynchronous interface so that we can make it
actually asynchronous in subsequent patches.

Create state structures and callback function signatures.  Add the
state structure to libxl__destroy_domid_state.  Break
libxl__destroy_domid down into two functions.

No functional change intended.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>