The storagePoolLookupByTargetPath() method in the storage driver is used
by the QEMU driver during block migration. If there's a valid use case
for this in the QEMU driver, then external apps likely have similar
needs. Exposing it in the public API removes the direct dependancy from
the QEMU driver to the storage driver.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>