vl.c: convert cpu_model to cpu type and set of global properties before machine_init()

All machines that support user specified cpu_model either call
cpu_generic_init() or cpu_class_by_name()/CPUClass::parse_features
to parse feature string and to get CPU type to create.

Which leads to code duplication and hard-codding default CPU model
within machine_foo_init() code. Which makes it impossible to
get CPU type before machine_init() is run.

So instead of setting default CPUs models and doing parsing in
target specific machine_foo_init() in various ways, provide
a generic data driven cpu_model parsing before machine_init()
is called.

in follow up per target patches, it will allow to:
  * define default CPU type in consistent/generic manner
    per machine type and drop custom code that fallbacks
    to default if cpu_model is NULL
  * drop custom features parsing in targets and do it
    in centralized way.
  * for cases of
      cpu_generic_init(TYPE_BASE/DEFAULT_CPU, "some_cpu")
    replace it with
      cpu_create(machine->cpu_type) || cpu_create(TYPE_FOO)
    depending if CPU type is user settable or not.
    not doing useless parsing and clearly documenting where
    CPU model is user settable or fixed one.

Patch allows machine subclasses to define default CPU type
per machine class at class_init() time and if that is set
generic code will parse cpu_model into a MachineState::cpu_type
which will be used to create CPUs for that machine instance
and allows gradual per board conversion.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1505318697-77161-4-git-send-email-imammedo@redhat.com>
Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>