block: Add idle_time_ns to BlockDeviceStats

This patch adds the new field 'idle_time_ns' to the BlockDeviceStats
structure, indicating the time that has passed since the previous I/O
operation.

It also adds the block_acct_idle_time_ns() call, to ensure that all
references to the clock type used for accounting are in the same
place. This will later allow us to use a different clock for iotests.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 7d8cfcf931453e1a2443e6626e8c1edc347c7c8a.1446044837.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qapi/block-core.json

@@ -448,6 +448,10 @@
 # @wr_merged: Number of write requests that have been merged into another
 #             request (Since 2.3).
 #
+# @idle_time_ns: #optional Time since the last I/O operation, in
+#                nanoseconds. If the field is absent it means that
+#                there haven't been any operations yet (Since 2.5).
+#
 # Since: 0.14.0
 ##
 { 'struct': 'BlockDeviceStats',
@@ -455,7 +459,7 @@
            'wr_operations': 'int', 'flush_operations': 'int',
            'flush_total_time_ns': 'int', 'wr_total_time_ns': 'int',
            'rd_total_time_ns': 'int', 'wr_highest_offset': 'int',
-           'rd_merged': 'int', 'wr_merged': 'int' } }
+           'rd_merged': 'int', 'wr_merged': 'int', '*idle_time_ns': 'int' } }
 
 ##
 # @BlockStats: