Move common logic in virt-launcher to virt-launcher-common pkg

[harshitgupta1337]

What this PR does

Before this PR: The logic in virt-launcher that is agnostic of the underlying virtualization stack (e.g., gRPC server for managing VM, DomainManager interface, NotifyClient) was in the same package as the virtualization-stack-specific code.

After this PR: Virtualization-stack-agnostic code has been moved to a separate package called virt-launcher-common. The libvirt/qemu/kvm-specific logic in virt-launcher is left in the virt-launcher package. virt-launcher-common does not depend on any code in the libvirt-specific virt-launcher package.

[JocelynBerrendonner]

This change is really massive. How can we stage these changes to prevent such a massive PR?

[harshitgupta1337]

This change is really massive. How can we stage these changes to prevent such a massive PR?

@JocelynBerrendonner I agree that the PR has a very large number of files that are changed. The vast majority of files updated simply have changes to import statements. This is because a lot of the common code in virt-launcher has been moved to virt-launcher-common. Since a lot of other code depends on that core part of virt-launcher's functionality, it necessitated import statement changes in a large number of files.
A potential alternative approach is to let the common parts of virt-launcher remain where they are currently and, instead, move the Libvirt/QEMU/KVM-specific code into another folder named virt-launcher-libvirt-qemu. Although it might also need a number of files to be moved, the PR footprint might be smaller.