interfaces/lxd: lxd slot implementation can also be an app snap

[jdstrand]

The lxd snap has existed for a while, but the interface mistakenly used 'reservedForOS: true'.

[chipaca]

oops

[codecov-io]

Codecov Report

Merging #4004 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4004      +/-   ##
==========================================
+ Coverage   75.71%   75.71%   +<.01%     
==========================================
  Files         424      424              
  Lines       36609    36608       -1     
==========================================
+ Hits        27718    27719       +1     
+ Misses       6945     6943       -2     
  Partials     1946     1946

Impacted Files	Coverage Δ
interfaces/builtin/lxd.go	100% <ø> (ø)	⬆️
overlord/ifacestate/helpers.go	63% <0%> (+0.66%)	⬆️

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[zyga]

LGTM

[stolowski]

Thanks!