virhostcpu: Make virHostCPUGetMSR() work only on x86

virhostcpu: Make virHostCPUGetMSR() work only on x86

Model specific registers are a thing only on x86. Also, the
/dev/cpu/0/msr path exists only on Linux and the fallback
mechanism (asking KVM) exists on Linux and FreeBSD only.

Therefore, move the function within #ifdef that checks all
aforementioned constraints and provide a dummy stub for all
other cases.

This fixes the build on my arm box, mingw-* builds, etc.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>