For AMD SEV determine C-bit position and guest mode in locore0.

For AMD SEV determine C-bit position and guest mode in locore0.

Actually determine the C-bit position if we are running as a guest
with SEV enabled.  Configure pg_crypt, pg_frame and pg_lgframe
accordingly, using the physical address bit reduction provided by
cpuid.

from hshoexer@; OK mlarkin@