tools/xenstore: add control command for setting and showing quota

Add a xenstore-control command "quota" to:
- show current quota settings
- change quota settings
- show current quota related values of a domain

Note that in the case the new quota is lower than existing one,
Xenstored may continue to handle requests from a domain exceeding the
new limit (depends on which one has been broken) and the amount of
resource used will not change. However the domain will not be able to
create more resource (associated to the quota) until it is back to below
the limit.

This is part of XSA-326.

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

docs/misc/xenstore.txt

@@ -410,6 +410,17 @@ CONTROL			<command>|[<parameters>|]
 	print|<string>
 		print <string> to syslog (xenstore runs as daemon) or
 		to console (xenstore runs as stubdom)
+	quota|[set <name> <val>|<domid>]
+		without parameters: print the current quota settings
+		with "set <name> <val>": set the quota <name> to new value
+		<val> (The admin should make sure all the domain usage is
+		below the quota. If it is not, then Xenstored may continue to
+		handle requests from the domain as long as the resource
+		violating the new quota setting isn't increased further)
+		with "<domid>": print quota related accounting data for
+		the domain <domid>
+	quota-soft|[set <name> <val>]
+		like the "quota" command, but for soft-quota.
 	help			<supported-commands>
 		return list of supported commands for CONTROL