vmx: Prefer consistent naming for loader tunables

[zlei]

Summary

The following loader tunables does have corresponding sysctl MIBs but
with inconsistent naming. That may be historical reason. Let's prefer
consistent naming for them so that they will be easier to read and
maintained.

1. hw.vmm.l1d_flush -> hw.vmm.vmx.l1d_flush
2. hw.vmm.l1d_flush_sw -> hw.vmm.vmx.l1d_flush_sw
3. hw.vmm.vmx.use_apic_pir -> hw.vmm.vmx.cap.posted_interrupts
4. hw.vmm.vmx.use_apic_vid -> hw.vmm.vmx.cap.virtual_interrupt_delivery
5. hw.vmm.vmx.use_tpr_shadowing -> hw.vmm.vmx.cap.tpr_shadowing

Old names are kept for compatibility.

Meanwhile, add sysctl flag CTLFLAG_TUN to them so that sysctl -T will
report them correctly.

MFC after: 1 week

Test Plan

On Intel x64 platform, load vmm kernel modules and verify the loader tunables.