tools: add example application to initialize dom0less PV drivers

tools: add example application to initialize dom0less PV drivers

Add an example application that can be run in dom0 to complete the
dom0less domains initialization so that they can get access to xenstore
and use PV drivers.

The application sets "connection" to XENSTORE_RECONNECT on the xenstore
page before calling xs_introduce_domain to signal that the connection is
not ready yet to be used. XENSTORE_RECONNECT is reset soon after by
xenstored.

Signed-off-by: Luca Miccio <lucmiccio@gmail.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Julien Grall <jgrall@amazon.com>
CC: Wei Liu <wl@xen.org>
CC: Anthony PERARD <anthony.perard@citrix.com>
CC: Juergen Gross <jgross@suse.com>