Add support for Xen/XCP-ng xe-guest-utilities

12.0 PR: https://github.com/freenas/build/pull/271

20.12 PR: https://github.com/freenas/build/pull/270

12.0 PR: https://github.com/freenas/ports/pull/873

20.12 PR: https://github.com/freenas/ports/pull/872

Note to TrueNAS developers: Installing xe-guest-utilities on TrueNAS 12.0-Release causes all kinds of problems (at least in my install, which I think is pretty much vanilla).

Installing xe-guest-utilities adds dependencies Python 3.7.9 and libffi 3.3.1 which may break TrueNAS 12.0.

Some Symptoms:

1) 4 minute hang on boot at Lo0: link state changed to UP

2) Missing RAIDZ2 Storage Pool (offline)

3) No guest utilities features present in XCP-ng even after installing the xe-guest-utilities pkg. 

4) Other boot related complications related to RAM and nested virtualization

See this bug report:

https://github.com/xcp-ng/xcp/issues/446

Finally, yes, it would be great if the xe-guest-utilities were built into TrueNAS and tested to avoid the above issues. 

One key benefit of xe-guest-utilities would be that nut-client running on dom0 of XCP-ng could send a graceful shutdown command to the TrueNAS VM in the event of a power outage. It is advantageous to have a single script running all VM shutdown routines rather than having each individual VM running nut-client.