network: allow "modify" option for DNS hostname

network: allow "modify" option for DNS hostname

The "modify" command allows you to replace an existing record
(its hostname, sub-elements). IP address acts as the primary key.
If it is not found, the attempt ends with an error message. If
the XML contains a duplicate address, it will select the last
one.

Tests in networkxml2xmlupdatetest.c contain replacements of an
existing DNS-Host record and failure due to non-existing record.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/639
Signed-off-by: Adam Julis <ajulis@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>