libxl: Refactor kill_device_model to libxl__kill_xs_path

libxl: Refactor kill_device_model to libxl__kill_xs_path

Move kill_device_model to libxl__kill_xs_path so we have a helper to
kill a process from a pid stored in xenstore.  We'll be using it to kill
vchan-qmp-proxy.

libxl__kill_xs_path takes a "what" string for use in printing error
messages.  kill_device_model is retained in libxl_dm.c to provide the
string.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>