shell: Polish create volume group dialog

Add warning when there are no available disks
Validate the length and content of input
Add visual style when error occurs
Check the precondition of creating a volume group

Fixes #871
It seems that the following functions are only ever used from within the ```enter()``` method. Could you make these nested functions:

 * check_vg_condition
 * change_disk_count
 * check_vg_input
 * check_input
 * change_checkbox_count
 * check_checked_box
 * count_checked_box
 * control_warning
 * hide_error
 * hide_error_message

These functions do not need to be callable on the object as methods.