libxenlight: separate logically list_vm and list_domain

libxenlight: separate logically list_vm and list_domain

previously list_domain was something between listing VM and domains.
provide 2 separates API calls to list domains and list vms.  the list
vms API filters utility domains like stubdomains, and domain 0

change is_stubdom to properly check the integer and also return a
boolean value.

Signed-off-by: Vincent Hanquez <vincent.hanquez@eu.citrix.com>