Fix localtime handling for Xen-PV domains

Fix localtime handling for Xen-PV domains

At least Xen-3.4.3 translates the /vm/localtime SXPR value to
/domain/platform/localtime and /domain/image/{linux,hvm}/localtime when
the domain is defined.  When reading back that information libvirt only
handles HVM domains, but not PV domains: This results in libvirtd always
returning
    <clock offset="utc"/>
while Xend used (localtime 1).

For PV domains use /domain/image/linux/localtime.