assumptions from generic code.

This implements the minimal set of changes needed in libvirt to launch a
PowerPC-KVM based guest.
It removes x86-specific assumptions about choice of serial driver backend
from generic qemu guest commandline generation code.
It also restricts the ACPI capability to be available for an x86 or
x86_64 domain.
This is not a complete solution -- it still does not guarantee libvirt
the capability to flag non-supported options in guest XML. (Eg, an ACPI
specification in a PowerPC guest XML will still get processed, even
though qemu-system-ppc64 does not support it while qemu-system-x86_64 does.)
This drawback exists because libvirt falls back on qemu to query supported
features, and qemu '-h' blindly lists all capabilities -- irrespective
of whether they are available while emulating a given architecture or not.
The long-term solution would be for qemu to list out capabilities based
on architecture and platform -- so that libvirt can cleanly make out what
devices are supported on an arch (say 'ppc64') and platform (say, 'mac99').
Signed-off-by: NPrerna Saxena <prerna@linux.vnet.ibm.com>