lxc-clone: maintain size of lvm snapshot
lxc-clone: maintain size of lvm snapshot

When creating a container as lvm snapshot, use the original size unless
user explicitly overrides it.
It's all well and good to day "use lvextend if you run out of space", but
in the meantime applications may become corrupted...

Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

ostap/lxc,ostap/lxc