KVM: x86: Explicitly #define the VCPU_REGS_* indices

Declaring the VCPU_REGS_* as enums allows for more robust C code, but it
prevents using the values in assembly files.  Expliciting #define the
indices in an asm-friendly file to prepare for VMX moving its transition
code to a proper assembly file, but keep the enums for general usage.

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