util/main-loop: Introduce the main loop into QOM

util/main-loop: Introduce the main loop into QOM

'event-loop-base' provides basic property handling for all 'AioContext'
based event loops. So let's define a new 'MainLoopClass' that inherits
from it. This will permit tweaking the main loop's properties through
qapi as well as through the command line using the '-object' keyword[1].
Only one instance of 'MainLoopClass' might be created at any time.

'EventLoopBaseClass' learns a new callback, 'can_be_deleted()' so as to
mark 'MainLoop' as non-deletable.

[1] For example:
      -object main-loop,id=main-loop,aio-max-batch=<value>

Signed-off-by: Nicolas Saenz Julienne <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Acked-by: Markus Armbruster <[email protected]>
Message-id: 20220425075723[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>