iotests/common.rc: Add _require_working_luks

That the luks driver is present is little indication on whether it is
actually working.  Without the crypto libraries linked in, it does not
work.  So add this function, which tries to create a luks image to see
whether that actually works.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200625125548.870061-4-mreitz@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

tests/qemu-iotests/common.rc

@@ -740,6 +740,33 @@ _unsupported_imgopts()
     done
 }
 
+# Caution: Overwrites $TEST_DIR/t.luks
+_require_working_luks()
+{
+    file="$TEST_DIR/t.luks"
+
+    output=$(
+        $QEMU_IMG create -f luks \
+            --object secret,id=sec0,data=hunter0 \
+            -o key-secret=sec0 \
+            -o iter-time=10 \
+            "$file" \
+            1M \
+            2>&1
+    )
+    status=$?
+
+    IMGFMT='luks' _rm_test_img "$file"
+
+    if [ $status != 0 ]; then
+        reason=$(echo "$output" | grep "$file:" | $SED -e "s#.*$file: *##")
+        if [ -z "$reason" ]; then
+            reason="Failed to create a LUKS image"
+        fi
+        _notrun "$reason"
+    fi
+}
+
 # this test requires that a specified command (executable) exists
 #
 _require_command()