Fix LVM volume listing with newer LVM

Per commit 0304f0ec, newer LVM has extended the lv_attr field. However,
that commit was incomplete as we examine this attribute in another
place in the code.

Thanks to user alperhome, the _LVSLINE_REGEX in lib/backend.py also
needs fixing. I've used the same change as in the above commit: accept
at minimum 6 characters, but allow for more.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>