scripts: auto-detect hypervisor in test-util.sh

Make the --hypervisor argument optional in process_common_args().
When it is not supplied, detect_hypervisor() now selects the backend
from the host device node: /dev/mshv for MSHV, /dev/kvm for KVM, and
errors out when neither is present. An explicit --hypervisor still
overrides the detection.

Assisted-by: Claude:Opus-4.8
Signed-off-by: Muminul Islam <muislam@microsoft.com>

scripts/test-util.sh

@@ -3,10 +3,23 @@
 # shellcheck source=/dev/null
 set -x
 
-hypervisor="kvm"
 test_filter=""
 build_kernel=false
 
+# Auto-detect the underlying hypervisor based on the device node exposed
+# by the host kernel: /dev/mshv for MSHV, /dev/kvm for KVM.
+# shellcheck disable=SC2034
+detect_hypervisor() {
+    if [ -e /dev/mshv ]; then
+        hypervisor="mshv"
+    elif [ -e /dev/kvm ]; then
+        hypervisor="kvm"
+    else
+        echo "ERROR: no hypervisor device found (/dev/mshv or /dev/kvm)" >&2
+        exit 1
+    fi
+}
+
 # Download from a url with retries
 # Args:
 #   $1: URL
@@ -98,7 +111,7 @@ cmd_help() {
     echo ""
     echo "Available arguments:"
     echo ""
-    echo "    --hypervisor  Underlying hypervisor. Options kvm, mshv"
+    echo "    --hypervisor  Underlying hypervisor. Options kvm, mshv. Auto-detected from the host device node when omitted."
     echo "    --test-filter Tests to run"
     echo "    --build-guest-kernel Build guest kernel from source instead of downloading pre-built"
     echo ""
@@ -140,8 +153,13 @@ process_common_args() {
         esac
         shift
     done
+    # Auto-detect the hypervisor when it was not provided explicitly.
+    if [ -z "$hypervisor" ]; then
+        detect_hypervisor
+    fi
     if [[ ! ("$hypervisor" = "kvm" || "$hypervisor" = "mshv") ]]; then
-        die "Hypervisor value must be kvm or mshv"
+        echo "ERROR: hypervisor must be kvm or mshv" >&2
+        exit 1
     fi
     # shellcheck disable=SC2034
     test_binary_args=("$@")