tools/xenstore: simplify xenstored main loop

tools/xenstore: simplify xenstored main loop

The main loop of xenstored is rather complicated due to different
handling of socket and ring-page interfaces. Unify that handling by
introducing interface type specific functions can_read() and
can_write().

Take the opportunity to remove the empty list check before calling
write_messages() because the function is already able to cope with an
empty list.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>