vl.c: error out if -mem-path is used together with -M memory-backend

the former is not actually used by explicit backend, so instead of
silently ignoring the option in non valid context, exit with error.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20200409134133.11339-1-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/softmmu/vl.c b/softmmu/vl.c
index 58a40bc..32c0047 100644
--- a/softmmu/vl.c
+++ b/softmmu/vl.c
@@ -4315,6 +4315,11 @@
                              "explicitly specified 'memory-backend' property");
+        if (mem_path) {
+            error_report("'-mem-path' can't be used together with"
+                         "'-machine memory-backend'");
+            exit(EXIT_FAILURE);
+        }
         ram_size = backend_size;