qemu: Implement a stub cpuArchDriver.baseline() handler for aarch64

qemu: Implement a stub cpuArchDriver.baseline() handler for aarch64

Openstack Nova calls virConnectBaselineCPU() during initialization
of the instance to get a full list of CPU features.
This patch adds a stub to aarch64-specific code to handle
this request (no actual work is done). That's enough to have
this stub with limited functionality because qemu/kvm backend
supports only 'host-passthrough' cpu mode on aarch64.

Signed-off-by: Oleg Strikov <oleg.strikov@canonical.com>