[libvirt] openvzGetType() leaks memory

[Matthias Bolte]

All other drivers return a static string from their getType() function
(conforming with the API documentation), but the OpenVZ driver does
strdup() a string, generating a leak:

static const char *openvzGetType(virConnectPtr conn ATTRIBUTE_UNUSED) {
    return strdup("OpenVZ");
}

Matthias