firecracker status(): add binary and KVM readiness checks

Adds `_firecracker_binary_ok()` (runs `firecracker --version`) and
`_kvm_accessible()` (opens /dev/kvm, issues KVM_GET_API_VERSION ioctl)
and gates `status()` on both, so the launch preflight reports missing
binary or inaccessible KVM before the caller ever attempts a boot.

Regression tests in TestFirecrackerBinaryCheck, TestFirecrackerKvmCheck,
and TestFirecrackerStatusRuntime cover all branches. Updated the
pre-existing TestFirecrackerStatus fixture to also stub the two new
helpers so it still exercises only the tap-pool/nft logic it was
designed for.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit/test_firecracker_backend.py

@@ -136,7 +136,9 @@ class TestFirecrackerStatus(unittest.TestCase):
         from bot_bottle.backend.firecracker import setup as fc_setup
         with patch.object(fc_setup.netpool, "missing_taps", return_value=[]), \
              patch.object(fc_setup.netpool, "overlapping_routes", return_value=[]), \
-             patch.object(fc_setup.shutil, "which", return_value=None):
+             patch.object(fc_setup.shutil, "which", return_value=None), \
+             patch.object(fc_setup, "_firecracker_binary_ok", return_value=True), \
+             patch.object(fc_setup, "_kvm_accessible", return_value=True):
             rc, out = self._run()
         self.assertEqual(0, rc)
         self.assertIn("unverified", out)