Right now when building the qemu command line, we try to do various
unconditional validations of the guest CPU against the host CPU. However
this checks are overly applied. The only time we should use the checks
are:

- The user requests host-model/host-passthrough, or

- When KVM is requsted. CPU features requested in TCG mode are always
  emulated by qemu and are independent of the host CPU, so no host CPU
  checks should be performed.

Right now if trying to specify a CPU for arm on an x86 host, it attempts
to do non-sensical validation and falls over.

Switch all the test cases that were intending to test CPU validation to
use KVM, so they continue to test the intended code.

Amend some aarch64 XML tests with a CPU model, to ensure things work
correctly.

On my machine, a guest fails to boot if it has a sound card, but not
graphical device/display is configured, because pulseaudio fails to
initialize since it can't access $HOME.

A workaround is removing the audio device, however on ARM boards there
isn't any option to do that, so -nographic always fails.

Set QEMU_AUDIO_DRV=none if no <graphics> are configured. Unfortunately
this has massive test suite fallout.

Add a qemu.conf parameter nographics_allow_host_audio, that if enabled
will pass through QEMU_AUDIO_DRV from sysconfig (similar to
vnc_allow_host_audio)

When launching a QEMU guest the binary is probed to discover
the list of supported CPU names. Remove this probing with a
simple lookup of CPU models in the qemuCapsPtr object. This
avoids another invocation of the QEMU binary during the
startup path.

As a nice benefit we can now remove all the nasty hacks from
the test suite which were done to avoid having to exec QEMU
on the test system. The building of the -cpu command line
can just rely on data we pre-populate in qemuCapsPtr.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

In case a hypervisor doesn't support the exact CPU model requested by a
domain XML, we automatically fallback to a closest CPU model the
hypervisor supports (and make sure we add/remove any additional features
if needed). This patch adds 'fallback' attribute to model element, which
can be used to disable this automatic fallback.