The dom0 plugin code had been using `pickle` for serializing input and
`json` for serializing output which was needlessly inconsistent. This
patch makes the code use `pickle`--chosen for its better handling of
`datetime` objects--for both sending and receiving data.
This patch also refactors the code so that neither the caller nor the
callee need to explicitly worry about serialization: the caller just
passes in args and kwargs, and the callee's function signature just
accepts the args and kwargs as usual.
Bonus: Removes unecessary imports
Change-Id: I3abb42eeebd8d37d67e6c26fa7bcae66d876b3ee
6392ad2924