fix #2046 add volume_size_info to LVMPlugin

fix #2046 add volume_size_info to LVMPlugin

Without volume_size_info a Storage plugin falls back to the Implementation
in PVE/Storage/Plugin.pm, which relies on `qemu-img info`.

`qemu-img info` returns wrong results on a node in the case of shared volume
groups (e.g. when sharing disks via iSCSI), if a disk was resized on another
node (it lseeks to the end of the block-device, and this yields the old size).

Using lvs directly fixes the issue, since the LVM metadata gets updated when
invoking lvs.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>