overlord/devicestate: deny parallel install of kernel or gadget snaps

[bboozzoo]

Deny parallel installation of kernel or gadget snaps.

[codecov-io]

Codecov Report

Merging #5587 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #5587      +/-   ##
=========================================
+ Coverage   79.09%   79.1%   +<.01%     
=========================================
  Files         515     515              
  Lines       39172   39174       +2     
=========================================
+ Hits        30983   30987       +4     
+ Misses       5691    5690       -1     
+ Partials     2498    2497       -1

Impacted Files	Coverage Δ
overlord/devicestate/devicestate.go	78.57% <100%> (+0.31%)	⬆️
overlord/hookstate/hookmgr.go	74.51% <0%> (+0.96%)	⬆️

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[chipaca]

so, we expect people to do parallel installs of core snaps?

[chipaca]

does what it says on the box.

But, it does make me wonder about core snaps.

[bboozzoo]

There's some TODO notes regarding core and snapd snaps left here still https://github.com/snapcore/snapd/blob/master/overlord/snapstate/snapstate.go#L90