KVM: VMX: Use RAX as the scratch register during vCPU-run

...to prepare for making the sub-routine callable from C code.  That
means returning the result in RAX.  Since RAX will be used to return the
result, use it as the scratch register as well to make the code readable
and to document that the scratch register is more or less arbitrary.

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