Create qubes.AppendLog service

[rootkovska]

Very useful to e.g. gather build logs in a reliable way, i.e. even if the build VM gets compromised during the build process (e.g due to curl|bash in a Makefile), the log can still be meaningful. Also for many other uses. Related to #830.

[andrewdavidwong]

HW42 has generously submitted a patch for this based on the proposal in the post Security challenges for the Qubes build process.