qmp: add filtering of statistics by target vCPU

Introduce a simple filtering of statistics, that allows to retrieve
statistics for a subset of the guest vCPUs.  This will be used for
example by the HMP monitor, in order to retrieve the statistics
for the currently selected CPU.

Example:
{ "execute": "query-stats",
  "arguments": {
    "target": "vcpu",
    "vcpus": [ "/machine/unattached/device[2]",
               "/machine/unattached/device[4]" ] } }

Extracted from a patch by Mark Kanda.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/monitor/stats.h

@@ -11,7 +11,7 @@
 #include "qapi/qapi-types-stats.h"
 
 typedef void StatRetrieveFunc(StatsResultList **result, StatsTarget target,
-                              Error **errp);
+                              strList *targets, Error **errp);
 typedef void SchemaRetrieveFunc(StatsSchemaList **result, Error **errp);
 
 /*
@@ -31,4 +31,13 @@ void add_stats_entry(StatsResultList **, StatsProvider, const char *id,
 void add_stats_schema(StatsSchemaList **, StatsProvider, StatsTarget,
                       StatsSchemaValueList *);
 
+/*
+ * True if a string matches the filter passed to the stats_fn callabck,
+ * false otherwise.
+ *
+ * Note that an empty list means no filtering, i.e. all strings will
+ * return true.
+ */
+bool apply_str_list_filter(const char *string, strList *list);
+
 #endif /* STATS_H */