libxl: remove code in stubdom creation failure path and callback

libxl: remove code in stubdom creation failure path and callback

The snippet to destroy stubdom and the callback were added in 1fc3aeb3
("libxl: use new QEMU xenstore protocol"). The intention was to destroy
stubdom when it is not responsive. That approach is problematic because
rc is not propagate back to sdss->callback, hence the guest is leaked.

The solution is simple. The destruction of stubdom can be done later in
sdss->callback. That code path already does the right thing to destroy
both the guest and the stubdom that serves the guest.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>