tools/hotplug: add wrapper to start xenstored

The shell wrapper in xenstored.service does not handle XENSTORE_TRACE.

Create a separate wrapper script which is used in the sysv runlevel
script and in systemd xenstored.service. It preserves existing
behaviour by handling the XENSTORE_TRACE boolean. It also implements
the handling of XENSTORED_ARGS=. This variable has to be added to
sysconfig/xencommons.

The wrapper uses exec unconditionally. This works because the
systemd service file passes --no-fork, which has the desired effect
that the binary launched by systemd becomes the final daemon
process. The sysv script does not pass --no-fork, which causes
xenstored to fork internally to return to the caller of the wrapper
script.

The place of the wrapper is currently LIBEXEC_BIN, it has to be
decided what the final location is supposed to be. IanJ wants it in
"/etc".

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>