libxl: fix device counting race in libxl__devices_destroy

libxl: fix device counting race in libxl__devices_destroy

Don't have a fixed number of devices in the aodevs array, and instead
size it depending on the devices present in xenstore.  Somewhat
formalise the multiple device addition/removal machinery to make this
clearer and easier to do.

As a side-effect we fix a few "lost thread of control" bug which would
occur if there were no devices of a particular kind.  (Various if
statements which checked for there being no devices have become
redundant, but are retained to avoid making the patch bigger.)

Specifically:

 * Users of libxl__ao_devices are no longer expected to know in
   advance how many device operations they are going to do.  Instead
   they can initiate them one at a time, between bracketing calls to
   "begin" and "prepared".

 * The array of aodevs used for this is dynamically sized; to support
   this it's an array of pointers rather than of structs.

 * Users of libxl__ao_devices are presented with a more opaque interface.
   They are are no longer expected to, themselves,
      - look into the array of aodevs (this is now private)
      - know that the individual addition/removal completions are
        handled by libxl__ao_devices_callback (this callback function
        is now a private function for the multidev machinery)
      - ever deal with populating the contents of an aodevs

 * The doc comments relating to some of the members of
   libxl__ao_device are clarified.  (And the member `aodevs' is moved
   to put it with the other members with the same status.)

 * The multidev machinery allocates an aodev to represent the
   operation of preparing all of the other operations.  See
   the comment in libxl__multidev_begin.

A wrinkle is that the functions are called "multidev" but the structs
are called "libxl__ao_devices" and "aodevs".  I have given these
functions this name to distinguish them from "libxl__ao_device" and
"aodev" and so forth by more than just the use of the plural "s"
suffix.

In the next patch we will rename the structs.

Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>