The form required a bridge member for an ethernet interface, which does
not use one. When the host's network devices could not be read, the error
was swallowed and every creation failed with "the device is not on this
host"; the page now shows the real error and does not try.
The IPv4 address and gateway were required even for DHCP or no IPv4, so
such an interface could not be created; malformed addresses passed and a
missing prefix crashed the XML builder. They are validated like the IPv6
fields now (netmask notation accepted, IPv6 scope ids rejected), and a
static address without a gateway gets no empty route. Interface names
follow the Linux rule (up to 15 letters, digits, . _ -). A bridge member,
or the device an ethernet interface configures, must be a device of the
host (the page no longer falls back to made-up names), a bridge cannot
contain itself, and needs STP and a non-negative delay.
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.
