docs: formatdomain: document <audio> element
docs: formatdomain: document <audio> element

Document the new <audio> element which allows to specify
host audio backend for a guest <sound> device, and update
the <sound> element description with the new <audio>
sub-element which specifies the other end of the mapping.

Signed-off-by: Roman Bogorodskiy <2a54bf90b2b53e2ab35ffe11dda2730a23b4aa9c@gmail.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

jardasgit/libvirt,libvirt/libvirt,olafhering/libvirt,crobinso/libvirt,libvirt/libvirt,olafhering/libvirt,jfehlig/libvirt,libvirt/libvirt,jardasgit/libvirt,nertpinx/libvirt,zippy2/libvirt,nertpinx/libvirt,crobinso/libvirt,nertpinx/libvirt,crobinso/libvirt,jardasgit/libvirt,jfehlig/libvirt,jfehlig/libvirt,crobinso/libvirt,jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,zippy2/libvirt,libvirt/libvirt,jardasgit/libvirt,olafhering/libvirt,zippy2/libvirt,nertpinx/libvirt,olafhering/libvirt,jardasgit/libvirt