KVM: SEV-ES: Set guest_state_protected after VMSA update

The refactoring in commit bb18a677 ("KVM: SEV: Acquire
vcpu mutex when updating VMSA") left behind the assignment to
svm->vcpu.arch.guest_state_protected; add it back.

Signed-off-by: Peter Gonda <pgonda@google.com>
[Delta between v2 and v3 of Peter's patch, which had already been
 committed; the commit message is my own. - Paolo]
Fixes: bb18a677 ("KVM: SEV: Acquire vcpu mutex when updating VMSA")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>