xenstored: send an evtchn notification on introduce_domain

xenstored: send an evtchn notification on introduce_domain

When xs_introduce_domain is called, send out a notification on the
xenstore event channel so that any (dom0less) domain waiting for the
xenstore interface to be ready can continue with the initialization.
Before sending the notification, clear XENSTORE_RECONNECTING.

The extra notification is harmless for domains that don't require it.

In xs_wire.h update the commment on top of XENSTORE_RECONNECTING to
generalize its meaning to suit the dom0less use-case better. Also
improve docs/misc/xenstore-ring.txt.

Signed-off-by: Luca Miccio <lucmiccio@gmail.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Juergen Gross <jgross@suse.com>