i386: kvm: filter CPUID feature words earlier, on cpu.c

cpu.c contains the code that will check if all requested CPU features
are available, so the filtering of KVM features must be there, so we can
implement "check" and "enforce" properly.

The only point where kvm_arch_init_vcpu() is called on i386 is:

- cpu_x86_init()
  - x86_cpu_realize() (after cpu_x86_register() is called)
    - qemu_init_vcpu()
      - qemu_kvm_start_vcpu()
        - qemu_kvm_thread_fn() (on a new thread)
          - kvm_init_vcpu()
            - kvm_arch_init_vcpu()

With this patch, the filtering will be done earlier, at:
- cpu_x86_init()
  - cpu_x86_register() (before x86_cpu_realize() is called)

Also, the KVM CPUID filtering will now be done at the same place where
the TCG CPUID feature filtering is done. Later, the code can be changed
to use the same filtering code for the "check" and "enforce" modes, as
now the cpu.c code knows exactly which CPU features are going to be
exposed to the guest (and much earlier).

One thing I was worrying about when doing this is that
kvm_arch_get_supported_cpuid() depends on kvm_irqchip_in_kernel(), and
maybe the 'kvm_kernel_irqchip' global variable wasn't initialized yet at
CPU creation time. But kvm_kernel_irqchip is initialized during
kvm_init(), that is called very early (much earlier than the machine
init function), and kvm_init() is already a requirement to run the
GET_SUPPORTED_CPUID ioctl() (as kvm_init() initializes the kvm_state
global variable).

Side note: it would be nice to keep KVM-specific code inside kvm.c. The
problem is that properly implementing -cpu check/enforce code (that's
inside cpu.c) depends directly on the feature bit filtering done using
kvm_arch_get_supported_cpuid(). Currently -cpu check/enforce is broken
because it simply uses the host CPU feature bits instead of
GET_SUPPORTED_CPUID, and we need to fix that.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
