KVM: VMX: Check guest support for RDTSCP before processing MSR_TSC_AUX

Check for RDTSCP support prior to checking if MSR_TSC_AUX is in the uret
MSRs array so that the array lookup and manipulation are back-to-back.
This paves the way toward adding a helper to wrap the lookup and
manipulation.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200923180409.32255-10-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>