guestagent contract for packages should be a list

the guestagent contract on the prepare call sends the packages to
install as a long space delimited string and this should be a list. The
other parameters are lists like users and databases. The packages should
be a list as well.

Closes-Bug: #1341966
Change-Id: I05233bfce6d4b81351b797a30d3a55313edf277e
Merge "guestagent contract for packages should be a list"