storage: allow opening with storage:///system and storage:///session URIs
storage: allow opening with storage:///system and storage:///session URIs

Allow the possibility of opening a connection to only the storage
driver, by defining storage:///system and storage:///session URIs
and registering a fake hypervisor driver that supports them.

The hypervisor drivers can now directly open a storage driver
connection at time of need, instead of having to pass around a
virConnectPtr through many functions. This will facilitate the later
change to support separate daemons for each driver.

Signed-off-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

libvirt/libvirt,andreabolognani/libvirt,zippy2/libvirt,libvirt/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,zippy2/libvirt,jardasgit/libvirt,libvirt/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,zippy2/libvirt,eskultety/libvirt,fabianfreyer/libvirt,olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt,jardasgit/libvirt,olafhering/libvirt,olafhering/libvirt,crobinso/libvirt,libvirt/libvirt,andreabolognani/libvirt,eskultety/libvirt,eskultety/libvirt,nertpinx/libvirt,jardasgit/libvirt,jfehlig/libvirt,andreabolognani/libvirt,eskultety/libvirt,nertpinx/libvirt,nertpinx/libvirt,olafhering/libvirt,jfehlig/libvirt,jardasgit/libvirt,crobinso/libvirt,jardasgit/libvirt,nertpinx/libvirt,zippy2/libvirt,eskultety/libvirt,crobinso/libvirt,crobinso/libvirt,nertpinx/libvirt