virDomainDiskDefGeometryParse: Use virXMLProp*
virDomainDiskDefGeometryParse: Use virXMLProp*

This strictens the parser to disallow negative values (interpreted as
`UINT_MAX + value + 1`) for attributes `cyls`, `heads` and `secs`.
Allowing negative numbers to be interpreted this way makes no sense for
these attributes.

Signed-off-by: Tim Wiederhake <da93ee425e056cfcf1ddac9fc87851b25464abf4@redhat.com>
Reviewed-by: Laine Stump <c23361c43fbf79fed83e8b76173707b083d6caf5@redhat.com>

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