Some hardware doesn't support portgroup fallback to single interface
fashion. This imposes additional restrictions on ports, for example
such ports will not support booting by PXE.
This patch adds restrictions setting pxe_enabled=True on ports that
are members of portgroups with standalone_ports_supported=False. And
vice versa portgroup.standalone_ports_supported can't be set to False
until portgroup contains ports with pxe_enabled=True.
Setting 'vif_port_id' means that we are using this port in standalone
mode. This patch also ensures that 'vif_port_id' can't be updated on
ports which are members of portgroups with
standalone_ports_supported=False. And vice versa updating portgroup
standalone_ports_supported=False is not allowed if portgroup contains
ports with extra/vif_port_id set.
Partial-bug: #1526403
Change-Id: I9b4682918725bed2da0b7c89666e2c37d8826290