First cut at backup agent to start backup sequence

Change-Id: Iec20cd1168d96cc4e558fc794d3509deeb8f754a
PEP 8 fixes
Aligned GuestAgent API for Prepare and Restore

Change-Id: I6dc9586c190fae593a5e28525a6941e7944aaf79
Fixing unittests

Change-Id: I57f67f1bb80f5d01b84917eb82bab1b3fc70f89d