Add "hw.vmm.topology.threads_per_core" and "hw.vmm.topology.cores_per_package"
tunables to modify the default cpu topology advertised by bhyve.

Also add a tunable "hw.vmm.topology.cpuid_leaf_b" to disable the CPUID
leaf 0xb. This is intended for testing guest behavior when it falls back
on using CPUID leaf 0x4 to deduce CPU topology.

The default behavior is to advertise each vcpu as a core in a separate soket.