stubdom/vtpm: make state save operation atomic

stubdom/vtpm: make state save operation atomic

This changes the save format of the vtpm stubdom to include two copies
of the saved data: one active, and one inactive. When saving the state,
data is written to the inactive slot before updating the key and hash
saved with the TPM Manager, which determines the active slot when the
vTPM starts up.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>