qemu: Validate memory access during validate domain config

Commit 6534b3c4 tried to raise an error when there is no numa
nodes by setting access='shared' in the domain config, but added
a helper called from qemuDomainDeviceDefValidate instead of a
helper called from qemuDomainDefValidate for XML:

<memoryBacking>
<hugepages/>
<access mode='shared'/>
</memoryBacking>

Since there are no memory devices in the test XML, there would
be no validation failure, but the test added was still failing.
Investigating that it turns out that unnecessary XML elements
were causing the failure (no need for <video>, <graphics>,
<pm>, usb controller model "piix3-uhci", disk attribute for
"discard='unmap'", <serial>, <console>, <channel> and a
memballoon model). Removing all those before moving the method
caused the test to succeed.

So this patch moves the validation to the right place and
removes all the unnecessary XML pieces that were causing
a false validation failure.

https://bugzilla.redhat.com/show_bug.cgi?id=1448149#c14

Signed-off-by: Luyao Huang <lhuang@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>