job: Add query-jobs QMP command

This adds a minimal query-jobs implementation that shouldn't pose many
design questions. It can later be extended to expose more information,
and especially job-specific information.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qapi/job.json

@@ -205,3 +205,49 @@
 # Since: 2.13
 ##
 { 'command': 'job-finalize', 'data': { 'id': 'str' } }
+
+##
+# @JobInfo:
+#
+# Information about a job.
+#
+# @id:                  The job identifier
+#
+# @type:                The kind of job that is being performed
+#
+# @status:              Current job state/status
+#
+# @current-progress:    Progress made until now. The unit is arbitrary and the
+#                       value can only meaningfully be used for the ratio of
+#                       @current-progress to @total-progress. The value is
+#                       monotonically increasing.
+#
+# @total-progress:      Estimated @current-progress value at the completion of
+#                       the job. This value can arbitrarily change while the
+#                       job is running, in both directions.
+#
+# @error:               If this field is present, the job failed; if it is
+#                       still missing in the CONCLUDED state, this indicates
+#                       successful completion.
+#
+#                       The value is a human-readable error message to describe
+#                       the reason for the job failure. It should not be parsed
+#                       by applications.
+#
+# Since: 2.13
+##
+{ 'struct': 'JobInfo',
+  'data': { 'id': 'str', 'type': 'JobType', 'status': 'JobStatus',
+            'current-progress': 'int', 'total-progress': 'int',
+            '*error': 'str' } }
+
+##
+# @query-jobs:
+#
+# Return information about jobs.
+#
+# Returns: a list with a @JobInfo for each active job
+#
+# Since: 2.13
+##
+{ 'command': 'query-jobs', 'returns': ['JobInfo'] }