target-i386: Set "check=off" by default on pc-*-2.4 and older

The default CPU model (qemu64) have some issues today: it enables some
features (ABM and SSE4a) that are not present in many host CPUs. That
means many hosts (but not all of them) had those features silently
disabled in the default configuration in QEMU 2.4 and older.

With the new "check=on" default, this causes warnings to be printed in
the default configuration, because of the lack of SSE4A on all Intel
hosts, and the lack of ABM on Sandy Bridge and older hosts:

  $ qemu-system-x86_64 -machine pc,accel=kvm
  warning: host doesn't support requested feature: CPUID.80000001H:ECX.abm [bit 5]
  warning: host doesn't support requested feature: CPUID.80000001H:ECX.sse4a [bit 6]

Those issues will be fixed in pc-*-2.5 and newer. But as we can't change
the guest ABI in pc-*-2.4, disable "check" mode by default in pc-*-2.4
and older so we don't print spurious warnings.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
