KVM: VMX: Don't save guest registers after VM-Fail

A failed VM-Enter (obviously) didn't succeed, meaning the CPU never
executed an instrunction in guest mode and so can't have changed the
general purpose registers.

In addition to saving some instructions in the VM-Fail case, this also
provides a separate path entirely and thus an opportunity to propagate
the fail condition to vmx->fail via register without introducing undue
pain.  Using a register, as opposed to directly referencing vmx->fail,
eliminates the need to pass the offset of 'fail', which will simplify
moving the code to proper assembly in future patches.

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