KVM: VMX: Consolidate to_vmx() usage in RFLAGS accessors

Capture struct vcpu_vmx in a local variable to improve the readability
of vmx_{g,s}et_rflags().

No functional change intended.

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