KVM: SEV: remove ghcb variable declarations

To avoid possible time-of-check/time-of-use issues, the GHCB should
almost never be accessed outside dump_ghcb, sev_es_sync_to_ghcb
and sev_es_sync_from_ghcb.  The only legitimate uses are to set the
exitinfo fields and to find the address of the scratch area embedded
in the ghcb.  Accessing ghcb_usage also goes through svm->sev_es.ghcb
in sev_es_validate_vmgexit(), but that is because anyway the value is
not used.

Removing a shortcut variable that contains the value of svm->sev_es.ghcb
makes these cases a bit more verbose, but it limits the chance of someone
reading the ghcb by mistake.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>