iotests/147: Fix drive parameters

8dff69b94 added an aio parameter to the drive parameter but forgot to
add a comma before, thus breaking the test.  Fix it again.

Fixes: 8dff69b9415b4287e900358744b732195e1ab2e2
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200206130812.612960-1-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

tests/qemu-iotests/147

@@ -134,7 +134,7 @@ class BuiltinNBD(NBDBlockdevAddBase):
         self.server.add_drive_raw('if=none,id=nbd-export,' +
                                   'file=%s,' % test_img +
                                   'format=%s,' % imgfmt +
-                                  'cache=%s' % cachemode +
+                                  'cache=%s,' % cachemode +
                                   'aio=%s' % aiomode)
         self.server.launch()