drivers: Implement virListAllDomains for drivers using virDomainObj

drivers: Implement virListAllDomains for drivers using virDomainObj

This patch adds support for listing all domains into drivers that use
the common virDomainObj implementation: libxl, lxc, openvz, qemu, test,
uml, vmware.

For drivers that don't support managed save images the guests are
treated as if they had none, so filtering guests that do have such an
image on this driver succeeds and produces 0 results.