cpu feature selection is not affected to guest 's cpuid with whpx

On windows with -accel whpx, "-cpu" is ignored without any messages.
Guest recognizes features as same as host's.

Confirmed on v5.2.0-rc1.

I suggest qemu may do,

- Warn with incompatible -cpu options were given.
- Enhance cpuid handling.

Background:
I was investigated mmio and block copy issue in Linux kernel.
I met a problem that Linux went ill for touching mmio with whpx. (not with tcg)
I suspect erms(enhanced rep movs) might trigger.
I tried to mask erms on qemu with -feature,erms, but it was ineffective.

At last, I disabled erms manually, to tweak whpx-all.c to mask erms in cpuid.

FYI, qemu with whpx from/to mmio, "rep movsb" does byte access regardless of erms.
Linux kernel tends to choose not "rep movsq" but "rep movsb" with erms.