pc: rename machine types

pc: rename machine types

Starting with release 1.4 we have a fully functional q35 machine type,
i.e. "qemu -M q35" JustWorks[tm].  Update machine type names to reflect
that:

  * pc-1.4 becomes pc-i440fx-1.4
  * q35-next becomes pc-q35-1.4

The pc-1.3 (+older) names are maintained for compatibility reasons.
For the same reason the "pc" and "q35" aliases are kept.  pc-piix-1.4
continues to be the default machine type, again for compatibility
reasons.

Also updated the description (shown by "qemu -M ?") with host bridge
name, south bridge name and chipset release year.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>