block/copy-before-write: implement cbw-timeout option

In some scenarios, when copy-before-write operations lasts too long
time, it's better to cancel it.

Most useful would be to use the new option together with
on-cbw-error=break-snapshot: this way if cbw operation takes too long
time we'll just cancel backup process but do not disturb the guest too
much.

Note the tricky point of realization: we keep additional point in
bs->in_flight during block_copy operation even if it's timed-out.
Background "cancelled" block_copy operations will finish at some point
and will want to access state. We should care to not free the state in
.bdrv_close() earlier.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
  [vsementsov: use bdrv_inc_in_flight()/bdrv_dec_in_flight() instead of
   direct manipulation on bs->in_flight]
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

qapi/block-core.json

@@ -4227,12 +4227,18 @@
 # @on-cbw-error: Behavior on failure of copy-before-write operation.
 #                Default is @break-guest-write. (Since 7.1)
 #
+# @cbw-timeout: Zero means no limit. Non-zero sets the timeout in seconds
+#               for copy-before-write operation. When a timeout occurs,
+#               the respective copy-before-write operation will fail, and
+#               the @on-cbw-error parameter will decide how this failure
+#               is handled. Default 0. (Since 7.1)
+#
 # Since: 6.2
 ##
 { 'struct': 'BlockdevOptionsCbw',
   'base': 'BlockdevOptionsGenericFormat',
   'data': { 'target': 'BlockdevRef', '*bitmap': 'BlockDirtyBitmap',
-            '*on-cbw-error': 'OnCbwError' } }
+            '*on-cbw-error': 'OnCbwError', '*cbw-timeout': 'uint32' } }
 
 ##
 # @BlockdevOptions: