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 <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>