pc: add 2.4 machine types

pc: add 2.4 machine types

The following patches will limit the following things to legacy
machine type:

- maximum number of virtqueues for virtio-pci were limited to 64
- auto msix bar size for virtio-net-pci were disabled by default

Cc: Paolo Bonzini <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Michael S. Tsirkin <[email protected]>
Signed-off-by: Jason Wang <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>