With qmp_send() handling variable length messages and string formatting
he callers of qmp_send() can be cleaned up to not care about these
things.

Drop the QMP_MSG_LEN sized buffers and use the message formatting, as
appropriate.
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Link: https://lore.kernel.org/r/20230811205839.727373-5-quic_bjorande@quicinc.comSigned-off-by: Bjorn Andersson <andersson@kernel.org>