Get specific available machines by label, tag and platform.

[cccs-kevin]

Thanks for contributing! But first: did you read our community guidelines?
https://cuckoo.sh/docs/introduction/community.html

What I have added/changed is:

I submit multiple files with a specific machine names, platforms or tags. I submit more files with these parameters than there are machines that match these characteristics in my Cuckoo system. I want these files that cannot be assigned to a machine right away to wait in the queue. They currently do not in v2.0.7 implementation of scheduler, and instead sit in a weird limbo state where they get picked off the queue and then waits for a machine to become available that matches these parameters.

This fix applies to the KVM machinery and label, platform and tags at the moment, by overriding the availables method and provides the option to get the count of specific available machines.

I feel like this should be implemented in the scheduler, but I can see the argument to have this implemented in self.db.fetch() as well. Let me know what you think!

The goal of my change is:

Fix the weird limbo state.

What I have tested about my change is:

Manual testing for correct functionality.