abstractconfig: add pending changes related helpers

abstractconfig: add pending changes related helpers

use a better naming scheme for methods from qemuserver:

split_flagged_list -> parse_pending_delete
join_flagged_list -> print_pending_delete
vmconfig_delete_pending_option -> add_to_pending_delete
vmconfig_undelete_pending_option -> remove_from_pending_delete
vmconfig_cleanup_pending -> cleanup_pending

parse_pending_delete now has a better representation of the force value,
which is encoded in a perl hash i.e. $conf->{$opt}->{force} = 1 or 0
depending on if the string in config has '!' or not.

Signed-off-by: Oguz Bektas <o.bektas@proxmox.com>