monitor: move json init from OPEN event to init

monitor: move json init from OPEN event to init

At some point in the past, the OPEN event was changed to be issued from a
bottom half.  This creates a small window whereas a data callback registered in
init may be invoked before the OPEN event has been issued.

This is reproducible with:

 echo "{'execute': 'qmp_capabilities'}" | qemu-system-x86_64 -M none -qmp stdio

We can fix this for the monitor by moving the parser initialization to init.

The remaining state that is set in OPEN appears harmless.

Reported-by: Daniel Berrange <berrange@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>