tools/xenstored: Introduce lu_get_connection() and use it

tools/xenstored: Introduce lu_get_connection() and use it

At the moment, dump_state_buffered_data() is taking two connections
in parameters (one is the connection to dump, the other is the
connection used to request LU). The naming doesn't help to
distinguish (c vs conn) them and this already lead to several mistake
while modifying the function.

To remove the confusion, introduce an help lu_get_connection() that
will return the connection used to request LU and use it
in place of the existing parameter.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Juergen Gross <jgross@suse.com>