tools/xenstore: split up send_reply()

tools/xenstore: split up send_reply()

Today send_reply() is used for both, normal request replies and watch
events.

Split it up into send_reply() and send_event(). This will be used to
add some event specific handling.

add_event() can be merged into send_event(), removing the need for an
intermediate memory allocation.

This is part of XSA-326.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
(cherry picked from commit 9bfde319dbac2a1321898d2f75a3f075c3eb7b32)