Add fallback implementation to new delete LVM2 PV warning dialog (#670171)

The new delete non-empty LVM2 Physical Volume warning dialog uses
Gtk::MessageDialog::get_message_area() in the display of Volume Group
members.  This function was new in gtkmm 2.22, released September 2010,
which is not available in a number of current distributions including:
Unbuntu 10.04 LTS, RHEL 6.3, SLES 11-SP2, Debian 6.0.

Implement fallback method to display the VG name and member PVs in the
warning dialog for when get_message_area() is not available.

Bug #670171 - Add LVM PV read-write support