vtpmmgr: use XSM label as vTPM kernel hash

vtpmmgr: use XSM label as vTPM kernel hash

Because there is not currently a method for the vTPM Manager to obtain a
build hash of a vTPM, use the hash of the vTPM's XSM label as a
substitute.  This allows the vTPM Manager to distinguish between vTPMs
intended to be paired with a hardware domain kernel (which cannot use
pv-grub) and vTPMs which are paired with a pv-grub domain and therefore
contain reliable measurements of the guest kernel in PCRs 4 and 5.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Campbell <ian.campbell@citrix.com>