Apologies for the enormous commit, but I don't see a way to split this
up nicely. In the vast majority of cases it's a simple change. A few
extra places can use TRY instead of manual error checking though. :^)
A SPICE agent communicates with the host OS to provide nifty features
like clipboard sharing :^)
This patch implements only plain-text clipboard sharing.
See: github.com/freedesktop/spice-protocol/blob/master/spice/vd_agent.h