sdn: dhcp: only consider subnets that have dhcp-range configured

sdn: dhcp: only consider subnets that have dhcp-range configured

If DHCP is enabled on a zone with subnets, but no subnet has a
dhcp-range configured, then starting a VM will fail because no IP can
be allocated. This patch fixes this by only considering subnets that
have a dhcp-range configured and only failing if there is at least one
subnet with a dhcp-range configured.

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
Reviewed-by: Stefan Lendl <s.lendl@proxmox.com>
Tested-by: Stefan Lendl <s.lendl@proxmox.com>
Signed-off-by: Stefan Lendl <s.lendl@proxmox.com>