Add property for IPv6 address ('ip6'). Build default value similarly to
IPv4 - common prefix + QID or Disp ID (for DispVMs).
This all is disabled unless 'ipv6' feature is enabled. It is inherited
from netvm (not template).
Even when enabled, VM may decide to not use it - or simply not support
it.
QubesOS/qubes-issues#718
Instead of old per-VM flag 'pci_strictreset', now implement this as
per-device flag using features. To not fail on particular device
assignment set 'pci-no-strict-reset/DEVICE-BDF' to True. For
example 'pci-no-strict-reset/00:1b.0'.
QubesOS/qubes-issues#2257
