Improve accuracy of beakerd metrics reporting

- unit tests for check_virtualisibility()
- beakerd metrics tests are now more stringent
- check_virtualisibility() now defined only on MachineRecipe
- check_virtualisibility() also checks host_requires
- all recipe queue metrics now defined only for MachineRecipe

Bug: 902659
Change-Id: I3d77209d8511cf8b34132be6e722dc479fea336e
Merge "Improve accuracy of beakerd metrics reporting" into release-0.11