Having only migratable flags reported by default on the "host" CPU model
is safer for the following reasons:

 * Existing users may expect "-cpu host" to be migration-safe, if they
   take care of always using compatible host CPUs, host kernels, and
   QEMU versions.
 * Users who don't care aboug migration and want to enable all features
   supported by the host kernel can simply change their setup to use
   migratable=no.

Without this change, people using "-cpu host" will stop being able to
migrate, because now "invtsc" is getting enabled by default.

We are not setting migratable=yes by default on all X86CPU subclasses,
because users should be able to get non-migratable features enabled if
they ask for them explicitly.

Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>