fix(interfaces): validate IPv6 address and gateway of host interfaces

The form checked both with re.match("^[0-9a-f./:]+|^$"), which tests
only how a value starts: "2001:db8::10/64'/><evil/>" passed and went
into the interface XML, which is built with f-strings, while upper-case
hex was refused. An address without a prefix passed too and then failed
with a ValueError when the XML was built.

The address is parsed with ipaddress.IPv6Interface and stored as
address/prefix (/128 when none is given), the gateway with
IPv6Address. A static configuration needs an address, and the XML gets
a route only when a gateway is given instead of gateway=''. The HTML
patterns accept upper-case hex.

Eight of the ten form and XML checks fail on the previous code.

vrtManager/interface.py

@@ -44,9 +44,10 @@ class wvmInterfaces(wvmConnect):
         if ipv6_type == "static":
             address, prefix = ipv6_addr.split("/")
             xml += f"""<protocol family='ipv6'>
-                        <ip address='{address}' prefix='{prefix}'/>
-                        <route gateway='{ipv6_gw}'/>
-                      </protocol>"""
+                        <ip address='{address}' prefix='{prefix}'/>"""
+            if ipv6_gw:
+                xml += f"""<route gateway='{ipv6_gw}'/>"""
+            xml += """</protocol>"""
         if itype == "bridge":
             xml += f"""<bridge stp='{stp}' delay='{delay}'>
                         <interface name='{netdev}' type='ethernet'/>