monitor: Split monitor_init in HMP and QMP function

monitor: Split monitor_init in HMP and QMP function

Instead of mixing HMP and QMP monitors in the same function, separate
the monitor creation function for both.

While in theory, one could pass both MONITOR_USE_CONTROL and
MONITOR_USE_READLINE before this patch and both flags would do
something, readline support is tightly coupled with HMP: QMP never feeds
its input to readline, and the tab completion function treats the input
as an HMP command. Therefore, this configuration is useless.

After this patch, the QMP path asserts that MONITOR_USE_READLINE is not
set. The HMP path can be used with or without MONITOR_USE_READLINE, like
before.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190613153405.24769-3-kwolf@redhat.com>
[Zero initialization of Monitor moved from monitor_data_init() to
callers]
Signed-off-by: Markus Armbruster <armbru@redhat.com>