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>

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