machine: Refactor smp_parse() in vl.c as MachineClass::smp_parse()

machine: Refactor smp_parse() in vl.c as MachineClass::smp_parse()

To make smp_parse() more flexible and expansive, a smp_parse function
pointer is added to MachineClass that machine types could override.

The generic smp_parse() code in vl.c is moved to hw/core/machine.c, and
become the default implementation of MachineClass::smp_parse. A PC-specific
function called pc_smp_parse() has been added to hw/i386/pc.c, which in
this patch changes nothing against the default one .

Suggested-by: Eduardo Habkost <[email protected]>
Signed-off-by: Like Xu <[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
Message-Id: <20190620054525[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>