libxenlight: check for early failures of qemu-dm

This patch makes xl create check whether qemu-dm has started
correctly, and causes it to fail immediately with appropriate errors
if not. There are other bugfixes too.

More specifically:

* libxl_create_device_model forks twice rather than once so that the
process which calls libxl does not end up being the actual parent
of qemu. That avoids the need for the qemu-dm process to be reaped
at some indefinite time in the future.

* The first fork generates an intermediate process which is
responsible for writing the qemu-dm pid to xenstore and then merely
waits to collect and report on qemu-dm's exit status during
startup. New arguments to libxl_create_device_model allow the
preservation of its pid so that a later call can check whether the
startup is successful.

* The core of this functionality (the double fork, waitpid, signal
handling and so forth) is abstracted away into a new facility
libxl_spawn_... in libxl_exec.c.

Consequential changes:

* libxl_wait_for_device_model now takes a callback function parameter
which is called repeatedly in the loop iteration and allows the
caller to abort the wait.

* libxl_exec no longer calls fork; there is a new libxl_fork.

* There is a hook to override waitpid, which will be necessary for
some callers.

Remaining problems and other issues I noticed or we found:

* The error handling is rather inconsistent still and lacking in
places.

* destroy_device_model can kill random dom0 processes (!)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>