KVM: VMX: Modify only RSP when creating a placeholder for guest's RCX

In the vCPU-run asm blob, the guest's RCX is temporarily saved onto the
stack after VM-Exit as the exit flow must first load a register with a
pointer to the vCPU's save area in order to save the guest's registers.
RCX is arbitrarily designated as the scratch register.

Since the stack usage is to (1)save host, (2)save guest, (3)load host
and (4)load guest, the code can't conform to the stack's natural FIFO
semantics, i.e. it can't simply do PUSH/POP.  Regardless of whether it
is done for the host's value or guest's value, at some point the code
needs to access the stack using a non-traditional method, e.g. MOV
instead of POP.  vCPU-run opts to create a placeholder on the stack for
guest's RCX (by adjusting RSP) and saves RCX to its place immediately
after VM-Exit (via MOV).

In other words, the purpose of the first 'PUSH RCX' at the start of
the vCPU-run asm blob  is to adjust RSP down, i.e. there's no need to
actually access memory.  Use 'SUB $wordsize, RSP' instead of 'PUSH RCX'
to make it more obvious that the intent is simply to create a gap on
the stack for the guest's RCX.

Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>