virInterfaceLinkParseXML: Use virXMLProp*
virInterfaceLinkParseXML: Use virXMLProp*

This strictens the parser to disallow negative values (interpreted as
`UINT_MAX + value + 1`) for attribute `speed`, which does not make sense for
a value measured in Mbits per second.

Signed-off-by: Tim Wiederhake <da93ee425e056cfcf1ddac9fc87851b25464abf4@redhat.com>
Reviewed-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>

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