libxl: poll: Use poller_get and poller_put for poller_app

libxl: poll: Use poller_get and poller_put for poller_app

This makes the code more regular.  We are going to want to do some
more work in poller_get and poller_put, which work also wants to be
done for poller_app.

Two very minor functional changes:

 * We call malloc an extra time since poller_app is now a pointer

 * ERROR_FAIL on poller_get failing for poller_app is generated in
   libxl_ctx_init rather than passed through by libxl_poller_init
   from libxl__pipe_nonblock.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Jim Fehlig <jfehlig@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Jim Fehlig <jfehlig@suse.com>