Fix treatment of percent as lvm lv size spec.

Percent is an alternative to a literal size. It is a fixed size request
expressed as a percentage of the vg's free space after all requests with
an explicit size have been allocated (but not grown). That means that a
request defined using percent can still be growable. Such a request will
use the size calculated from the percent argument as the basis for growth
rate.

(cherry picked from commit e0e00f38320bb0b40fb7d8c3e08be2a6fdf5b54e)

Related: rhbz#1116435