qubesd: reduce verbosity
qubesd: reduce verbosity

Remove debug prints, log full traceback (of handled exception) only when
debug mode enabled (--debug, introduce in this commit too).
--debug option also enables sending tracebacks to the API clients.

QubesOS/qubes-issues#853

woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin