Don't start GUI daemon for given VM twice when qvm-start-gui was started
during VM startup (while waiting for qrexec startup). This is especially
common while running tests.
Report failed qubes.SetMonitorLayout as warning (instead of unhandled
exception).
Clear VM cache on qubesd reconnect.
Fix logging.
Since we use qubes.VMShell service now and send requested command on its
stdin, we need to terminate that shell after requested command -
otherwise the service will not terminate automatically waiting for
further input (next commands).
qubesd_call expect "bytes" type. Additionally serialize false value as
empty string (which is treated by python as false value), because
otherwise would be serialized to (non-empty) string, which is true value
in python.
