pc: Eliminate pc_default_machine_options()

pc: Eliminate pc_default_machine_options()

The only PC machines that didn't call pc_default_machine_options() were
isaps and xenfv. Both were already overwriting max_cpus, and only isapc
was not overwriting hot_add_cpu.

After making isapc set hot_add_cpu to NULL, we can move the
pc_default_machine_options() code the PC common class_init.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>