KVM: nVMX: Use descriptive names for VMCS sync functions and flags

Nested virtualization involves copying data between many different types
of VMCSes, e.g. vmcs02, vmcs12, shadow VMCS and eVMCS.  Rename a variety
of functions and flags to document both the source and destination of
each sync.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>