qemu: monitor: Refactor and fix monitor checking

qemu: monitor: Refactor and fix monitor checking

Among all the monitor APIs some where checking if mon is NULL and some
were not. Since it's possible to have mon equal to NULL in case a second
call is attempted once entered the monitor. This requires that every
single API checks for the monitor.

This patch adds a macro that helps checking the state of the monitor and
either refactors existing checking code to use the macro or adds it in
case it was missing.