vmx: Prefer consistent naming for loader tunables

The following loader tunables do have corresponding sysctl MIBs but
with different names. That may be historical reason. Let's prefer
consistent naming for them so that it will be easier to read and
maintain.

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.

Reviewed by: corvink, jhb, kib, bhyve
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D42251