i386: Don't set CPUClass::cpu_def on "max" model

Host CPUID info is used by the "max" CPU model only in KVM mode.
Move the initialization of CPUID data for "max" from class_init
to instance_init, and don't set CPUClass::cpu_def for "max".

Message-Id: <20170222183919.11928-4-ehabkost@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>