lvmstrap: ignore small-sized partitions

This patch changes lvmstrap to ignore “small” partitions. Currently
extended partitions are reported as unused as with a size of 1024
(bytes), and this confuses lvmstrap. Since a very small partition
won't help anyway (below hundred of PE size is not helpful), let's
restrict it to 1GB.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>