Fix #2109: resize rbd volume for container failed

Fix #2109: resize rbd volume for container failed

On resizing a container's disk image the filesystem is extended and in
the case of RBD the returned path of the volume was not a path to a
mapped device.

This patch uses map_volume (respectively unmap_volume) to get a device
mapped and its path returned by the storage plugin. If a path is not
returned then the path method is tried. Currently only the RBD storage
plugin returns a path on map_volume.

Signed-off-by: Alwin Antreich <a.antreich@proxmox.com>