fix(transfer): actually drop a RHEL guest's IPv6 on a host that has none

A container moving to a host with no IPv6 has to lose the address it carried,
and on RHEL-family guests that means telling NetworkManager, not editing a file.
NetworkManager will not accept the change in the obvious order — a method of
"manual" requires an address, and an address is not allowed under a method of
"disabled" — so the guest still came up with the address from the host it left.
Setting the whole final state in one request is what it accepts.

Verified with an AlmaLinux 9 guest in both directions against a prefix-mode and
a pool-mode host: moved to the prefix host it came back up with one bridged NIC
carrying this host's IPv4 and IPv6 (which required re-enabling the IPv4 that the
pool-mode layout had disabled on that NIC); moved to the pool host it came up
pure IPv4 with ipv6.method disabled and no global address at all.

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

src/internal/mgr/transfer.go

@@ -567,7 +567,11 @@ if command -v nmcli >/dev/null 2>&1 && ! systemctl is-active systemd-networkd >/
     CONN=$(nmcli -t -f NAME,DEVICE con show 2>/dev/null | awk -F: '$2 == "eth0" {print $1; exit}')
     [ -z "$CONN" ] && CONN=$(nmcli -t -f NAME con show 2>/dev/null | grep -i eth0 | head -1)
     if [ -n "$CONN" ]; then
-      nmcli con mod "$CONN" ipv6.method disabled >/dev/null 2>&1 || true
+      # The address and the method have to be cleared together: NetworkManager
+      # refuses a method of "manual" with no address left, and refuses an
+      # address under a method of "disabled", so only the final state, set in
+      # one request, is accepted.
+      nmcli con mod "$CONN" ipv6.method disabled ipv6.addresses "" ipv6.gateway "" >/dev/null 2>&1 || true
       nmcli con up "$CONN" >/dev/null 2>&1 || true
     fi
   fi