conf: Check for user aliases duplicates only

conf: Check for user aliases duplicates only

https://bugzilla.redhat.com/show_bug.cgi?id=1553162

When validating a device XML config we check if user provided
alias is unique. We do this by maintaining a hash table of device
aliases as we iterated over all devices defined for the domain.
However, it may happen that what appears as two devices in domain
XML is in fact just one interface in hypervisor.  We can assume
libvirt generated aliases to be unique and thus really check user
provided ones only.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>