Do not close event loop in utility function - handle it only in main().
For this reason, change appropriate functions to coroutines.
FixesQubesOS/qubes-issues#2865
qubesd may be restarted during different stages of connection - either
while attempting to connect, or while already listening on events.
Adjust exception list accordingly.
This is especially important for qvm-start-gui - otherwise it crashes on
qubesd restart.
Empty parameter value is encoded as b'parameter\0\0', so we can't simply
read the data until b'\0\0', because it isn't necessary event end.
Instead, read event parts separately, according to specification.
