iotests: Wait for qemu to end in 223

When iotest 223 was first written, it didn't matter if we waited for
the qemu process to clean up. But with the introduction of a later
qemu-nbd process trying to reuse the same file, there is a race where
even though the asynchronous qemu process has responded to "quit", it
has not yet had time to unlock the file and exit, resulting in:

-[{ "start": 0, "length": 65536, "depth": 0, "zero": false, "data": false},
-{ "start": 65536, "length": 2031616, "depth": 0, "zero": false, "data": true},
-{ "start": 2097152, "length": 2097152, "depth": 0, "zero": false, "data": false}]
+qemu-nbd: Failed to blk_new_open 'tests/qemu-iotests/scratch/t.qcow2': Failed to get shared "write" lock
+Is another process using the image [tests/qemu-iotests/scratch/t.qcow2]?
+qemu-img: Could not open 'driver=nbd,server.type=unix,server.path=tests/qemu-iotests/scratch/qemu-nbd.sock,x-dirty-bitmap=qemu:dirty-bitmap:b': Failed to connect socket tests/qemu-iotests/scratch/qemu-nbd.sock: Connection refused
+./common.nbd: line 33: kill: (11122) - No such process

Fixes: ddd09448
Reported-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190305182908.13557-1-eblake@redhat.com>
Tested-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>