Default video type to 'vga' for PowerKVM

PowerKVM currently doesn't support 'cirrus' video type. For now, we can
detect if Nova compute is running on ppc64 or ppc architecture systems
and set the default to 'vga', which is supported on PowerKVM.

This patch did some reordering of the logic in
nova.virt.libvirt.driver.LibvirtDriver.get_guest_config in an attempt to
set the video.type depending on the compute host architecture and
implementation specifics. The video.type can still be set using
image_meta.

Change-Id: I49a1a8e6d9ebdc28d876d2f958c8cb960892f322
Closes-Bug: 1274367
Merge "Default video type to 'vga' for PowerKVM"