tools/osbuilder/tests: Remove TRAVIS variable

The last remaining user of the TRAVIS variable in this repo is
tools/osbuilder/tests and it is only used to skip spinning up VMs. Travis
didn't support virtualization and the same is true for github actions hosted
runners. Replace the variable with KVM_MISSING and determine availability of
/dev/kvm at runtime.

TRAVIS is also used by '.ci/setup.sh' in kata-containers/tests to reduce the
set of dependencies that gets installed, but this is also in the process of
being removed.

Fixes: #3544
Signed-off-by: Jeremi Piotrowski <jpiotrowski@microsoft.com>

.github/workflows/static-checks.yaml

@@ -19,7 +19,6 @@ jobs:
           - "make test"
           - "sudo -E PATH=\"$PATH\" make test"
     env:
-      TRAVIS: "true"
       RUST_BACKTRACE: "1"
       target_branch: ${{ github.base_ref }}
     steps: