scripts: let the runtime adapter's suite run on KVM on a host that also has /dev/mshv #1019

The runtime adapter's suite command (command_test_runtime_adapter in scripts/nvx_tools/runtime_adapter_tests.py) checks its backend with the shared validate_openvmm_test_backend (scripts/nvx_tools/ci.py), which refuses kvm whenever /dev/mshv exists, because OpenVMM would otherwise select MSHV on its own. The runtime adapter names the hypervisor explicitly, though: its tests pass --hypervisor from NVX_HYPERVISOR (guest_options in tests/runtime-adapter/tests/common/mod.rs, and tests/runtime-adapter/src/layered.rs). So on a host that exposes both /dev/kvm and /dev/mshv, the suite refuses KVM although it could run on it.

To fix: separate the auto-selection restriction from the OS and device-access checks, and have the runtime adapter's command use only the latter.

Found in #963 (a review thread on scripts/nvx_tools/runtime_adapter_tests.py, line 275, still open at merge). Related to #833.