xenstore: add support for changing log functionality dynamically

xenstore: add support for changing log functionality dynamically

Today Xenstore supports logging only if specified at start of the
Xenstore daemon. As it can't be disabled during runtime it is not
recommended to start xenstored with logging enabled.

Add support for switching logging on and off at runtime and to
specify a (new) logfile. This is done via the XS_CONTROL wire command
which can be sent with xenstore-control.

To switch logging on just use:

xenstore-control log on

To switch it off again:

xenstore-control log off

To specify a (new) logfile:

xenstore-control logfile <file>

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>