tests/qemu-iotests: Restrict test 156 to the 'file' protocol

The test fails completely when you try to use it with a different
protocol, e.g. with "./check -ssh -qcow2 156".
The test uses some hand-crafted JSON statements which cannot work with other
protocols, thus let's change this test to only support the 'file' protocol.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20240315111108.153201-7-thuth@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/156

@@ -50,7 +50,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 . ./common.qemu
 
 _supported_fmt qcow2 qed
-_supported_proto generic
+_supported_proto file
 # Copying files around with cp does not work with external data files
 _unsupported_imgopts data_file