monitor: unify global init

There are many places where the monitor initializes its globals:

- monitor_init_qmp_commands() at the very beginning
- single function to init monitor_lock
- in the first entry of monitor_init() using "is_first_init"

Unify them a bit.

monitor_lock is not used before monitor_init() (as confirmed by code
analysis and gdb watchpoints); so we are safe delaying what was a
constructor-time initialization of the mutex into the later first call
to monitor_init().

Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-8-peterx@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>