VMs should not have vCPU-many sockets

this is a bit silly:

debian@debian-11:~$ likwid-topology -g
--------------------------------------------------------------------------------
CPU name:	AMD EPYC 7713P 64-Core Processor
CPU type:	AMD K19 (Zen3) architecture
CPU stepping:	1
********************************************************************************
Hardware Thread Topology
********************************************************************************
Sockets:		8
Cores per socket:	1
Threads per core:	1

I (and I think everyone else) thought we're emulating a single-socket machine until we intend otherwise, with vCPU-many cores and one or two threads per core. Looking into #921 I noticed we discard leaf Ext1E and then never repopulate it for the VM's CPU count, but we never clear the TopologyExtensions bit, so the all-zero leftovers is declared to be faithful. That gets us vCPU-many sockets, oops.

RFD 314 and https://github.com/oxidecomputer/omicron/pull/8728 in particular set leaf 8000_001E to have two threads per core, but I'm actually not sure that gets us to one socket with vCPU-many-(or vCPU/2) cores. Either way, out of the box Propolis should reconstruct the CPUID leaf StdB/Ext1E that it very eagerly discards.

This will change the CPUID profile of guests as-is, but I think that's good - I'm a little surprised that OSes are as nice about this as they are!