iotests: Enhance 223, 233 to cover 'qemu-nbd --list'

Any good new feature deserves some regression testing :)
Coverage includes:
- 223: what happens when there are 0 or more than 1 export,
proof that we can see multiple contexts including qemu:dirty-bitmap
- 233: proof that we can list over TLS, and that mix-and-match of
plain/TLS listings will behave sanely

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20190117193658.16413-22-eblake@redhat.com>

tests/qemu-iotests/223.out

@@ -30,12 +30,32 @@ wrote 2097152/2097152 bytes at offset 2097152
 {"error": {"class": "GenericError", "desc": "NBD server not running"}}
 {"return": {}}
 {"error": {"class": "GenericError", "desc": "NBD server already running"}}
+exports available: 0
 {"return": {}}
 {"error": {"class": "GenericError", "desc": "Cannot find device=nosuch nor node_name=nosuch"}}
 {"error": {"class": "GenericError", "desc": "NBD server already has export named 'n'"}}
 {"error": {"class": "GenericError", "desc": "Enabled bitmap 'b2' incompatible with readonly export"}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'b3' is not found"}}
 {"return": {}}
+exports available: 2
+ export: 'n'
+  size:  4194304
+  flags: 0x4ef ( readonly flush fua trim zeroes df cache )
+  min block: 512
+  opt block: 4096
+  max block: 33554432
+  available meta contexts: 2
+   base:allocation
+   qemu:dirty-bitmap:b
+ export: 'n2'
+  size:  4194304
+  flags: 0x4ed ( flush fua trim zeroes df cache )
+  min block: 512
+  opt block: 4096
+  max block: 33554432
+  available meta contexts: 2
+   base:allocation
+   qemu:dirty-bitmap:b2
 
 === Contrast normal status to large granularity dirty-bitmap ===