Lazy initialize the cache from querying LVM2 PVs (#160787)

Previously when GParted was started LVM2_PV_Info cache was loaded twice,
executing LVM2 PV querying commands twice.  Firstly when
lvm2_pv::get_filesystem_support() was checking if LVM2 PV support was
available, and secondly when forced by a refresh in
GParted_Core::set_devices().

Implement lazy initialization.  Only load the cache when forced by the
above mentioned refresh or having to return a value when the cache is
not yet loaded.  Do not initialize the cache when just checking if LVM2
PV support is available.

Bug #160787 - lvm support