KVM: SEV: Drop GHCB_VERSION_DEFAULT and open code it

KVM: SEV: Drop GHCB_VERSION_DEFAULT and open code it

Remove the GHCB_VERSION_DEFAULT macro and open code it with '2'. The macro
is used conditionally and is not a true default. KVM ABI does not
advertise/emumerates the default GHCB version. Any future change to this
macro would silently alter the ABI and potentially break existing
deployments that rely on the current behavior.

Additionally, move the GHCB version assignment earlier in the code flow and
update the comment to clarify that KVM_SEV_INIT2 defaults to version 2,
while KVM_SEV_INIT forces version 1.

No functional change intended.

Cc: Thomas Lendacky <thomas.lendacky@amd.com>
Cc: Michael Roth <michael.roth@amd.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Link: https://lore.kernel.org/r/20250819234833.3080255-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>