So far we have two arguments that we are passing to
qemuBuildMemoryBackendProps() and that are taken from domain
private data: @qemuCaps and @autoNodeset. In the next commit I
will use one more item from there. Therefore, instead of having
it as yet another argument to the function, pass pointer to the
private data object.

There is one change in qemuDomainAttachMemory() where previously
@autoNodeset was NULL but now is priv->autoNodeset (which may be
set). This is safe to do as @autoNodeset is advisory only.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>