pc: Define machines using a DEFINE_PC_MACHINE macro

This will automatically generate the existing QEMUMachine structs based
on the *_MACHINE_OPTIONS macros, and automatically add registration code
for them.

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>