nbd: reconnect-on-open feature

v2: simple fix for mypy and pylint complains on patch 04
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Merge tag 'pull-nbd-2021-12-22-v2' of https://src.openvz.org/scm/~vsementsov/qemu into staging

nbd: reconnect-on-open feature
  v2: simple fix for mypy and pylint complains on patch 04

# gpg: Signature made Thu 23 Dec 2021 12:45:20 AM PST
# gpg:                using RSA key 8B9C26CDB2FD147C880E86A1561F24C1F19F79FB
# gpg: Good signature from "Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>" [unknown]
# gpg: WARNING: This key is not certified with a trusted signature!
# gpg:          There is no indication that the signature belongs to the owner.
# Primary key fingerprint: 8B9C 26CD B2FD 147C 880E  86A1 561F 24C1 F19F 79FB

* tag 'pull-nbd-2021-12-22-v2' of https://src.openvz.org/scm/~vsementsov/qemu:
  iotests: add nbd-reconnect-on-open test
  iotests.py: add qemu_io_popen()
  iotests.py: add and use qemu_io_wrap_args()
  iotests.py: add qemu_tool_popen()
  nbd/client-connection: improve error message of cancelled attempt
  nbd/client-connection: nbd_co_establish_connection(): return real error
  nbd: allow reconnect on open, with corresponding new options

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

qapi/block-core.json

@@ -4096,6 +4096,12 @@
 #                   future requests before a successful reconnect will
 #                   immediately fail. Default 0 (Since 4.2)
 #
+# @open-timeout: In seconds. If zero, the nbd driver tries the connection
+#                only once, and fails to open if the connection fails.
+#                If non-zero, the nbd driver will repeat connection attempts
+#                until successful or until @open-timeout seconds have elapsed.
+#                Default 0 (Since 7.0)
+#
 # Features:
 # @unstable: Member @x-dirty-bitmap is experimental.
 #
@@ -4106,7 +4112,8 @@
             '*export': 'str',
             '*tls-creds': 'str',
             '*x-dirty-bitmap': { 'type': 'str', 'features': [ 'unstable' ] },
-            '*reconnect-delay': 'uint32' } }
+            '*reconnect-delay': 'uint32',
+            '*open-timeout': 'uint32' } }
 
 ##
 # @BlockdevOptionsRaw: