emulators/qemu: include zstd in tools flavor

[dch]

Summary

qemu-img convert builds significantly smaller images when using zstd compression,
even compared to default compression. 9GiB raw yields 6GiB compressed vs 1.6GiB
with zstd.

Approved by: bofh
Sponsored by: SkunkWerks, GmbH

[dch]

hi Moin, I'd like this for releng so we only need emulators/qemu-tools on the builder machines. It allows us to
reduce size of generated OCI images with qcow2 significantly - from 6GiB with default compression down to 1.6GiB with zstd:

qemu-img convert -S 512b -p -O qcow2 -c -o compression_type=zstd start.raw end.qcow2

atm zstd is only included in the nox11 and larger flavours, we dont want the graphical toolbox along for the ride.

[bofh]

I will take care of it during my weekend port rampage.

[bofh]

Fixed at https://cgit.freebsd.org/ports/commit/?id=ba7961f454325f729d11785e2f4a12ddff5b0f33

[dch]

already commited