target-i386: cpu: fix --disable-kvm compilation

target-i386: cpu: fix --disable-kvm compilation

This fixes the following:
  target-i386/cpu.o: In function `kvm_cpu_fill_host':
  target-i386/cpu.c:783: undefined reference to `kvm_state'

I didn't notice the problem before because GCC was optimizing the entire
kvm_cpu_fill_host() function out (because all calls are conditional on
kvm_enabled()).

* cpu_x86_fill_model_id() is used only if CONFIG_KVM is set, so #ifdef it
  entirely to avoid compiler warnings.

* kvm_cpu_fill_host() should be called only if KVM is enabled, so
  use #ifdef CONFIG_KVM around the entire function body.

Reported-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>