pseries: Instantiate USB interface when required

pseries: Instantiate USB interface when required

The pseries machine already supports the -vga std option, creating a
graphics adapter.  However, this is not very useful without being able to
add a keyboard and mouse as well.  This patch addresses this by adding
a USB interface when requested, and automatically adding a USB keyboard
and mouse when VGA is enabled.

This is a stop gap measure to get usable graphics mode on pseries while
waiting for Li Zhang's rework of USB options to go in after 1.2.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>