GuestHelpers: add tag related helpers

GuestHelpers: add tag related helpers

'get_allowed_tags':
returns the allowed tags for the given user

'assert_tag_permissions'
helper to check permissions for tag setting/updating/deleting
for both container and qemu-server

gets the list of allowed tags from the DataCenterConfig and the current
user permissions, and checks for each tag that is added/removed if
the user has permissions to modify it

'normal' tags require 'VM.Config.Options' on '/vms/<vmid>', but not
allowed tags (either limited with 'user-tag-access' or
'privileged-tags' in the datacenter.cfg) requrie 'Sys.Modify' on '/'

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>