KVM: nSVM: Add VMLOAD/VMSAVE helper to deduplicate code

Add another helper layer for VMLOAD+VMSAVE, the code is identical except
for the one line that determines which VMCB is the source and which is
the destination.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210205005750.3841462-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>