According to the QEMU Machine Protocol Specification, the messages sent
by QMP as a response to a command can be of two types: either an error
message (identified by the "error" key), or a success message
(identified by the "return" key).

Since we explicitly handle errors by raising a HypervisorError, there is
no reason to return the whole response message; instead we return only
the response part, which is the dual behavior of accepting commands from
the callers without the surrounding {"execute": ...} object.

We also unexport the RETURN_KEY constant, which is now only used
internally.

Signed-off-by: Apollon Oikonomopoulos <apoikos@gmail.com>
Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>