monitor: do not use QTAILQ_FOREACH_SAFE across critical sections

monitor: do not use QTAILQ_FOREACH_SAFE across critical sections

monitor_qmp_requests_pop_any_with_lock cannot modify the monitor list
concurrently with monitor_cleanup, since the dispatch bottom half
runs in the main thread, but anyway it is a bit ugly to keep
"next" live across critical sections of monitor_lock and Coverity
complains (CID 1397072).

Replace QTAILQ_FOREACH_SAFE with a while loop and QTAILQ_FIRST,
it is cleaner and more future-proof.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>