KVM: nSVM: rename functions and variables according to vmcbXY nomenclature

Now that SVM is using a separate vmcb01 and vmcb02 (and also uses the vmcb12
naming) we can give clearer names to functions that write to and read
from those VMCBs.  Likewise, variables and parameters can be renamed
from nested_vmcb to vmcb12.

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