error: Replace qemu_error() by error_report()

error_report() terminates the message with a newline.  Strip it it
from its arguments.

This fixes a few error messages lacking a newline:
net_handle_fd_param()'s "No file descriptor named %s found", and
tap_open()'s "vnet_hdr=1 requested, but no kernel support for
IFF_VNET_HDR available" (all three versions).

There's one place that passes arguments without newlines
intentionally: load_vmstate().  Fix it up.
diff --git a/qerror.c b/qerror.c
index a418bde..a6f0213 100644
--- a/qerror.c
+++ b/qerror.c
@@ -318,13 +318,13 @@
  * qerror_print(): Print QError data
  *
  * This function will print the member 'desc' of the specified QError object,
- * it uses qemu_error() for this, so that the output is routed to the right
+ * it uses error_report() for this, so that the output is routed to the right
  * place (ie. stderr or Monitor's device).
  */
 void qerror_print(const QError *qerror)
 {
     QString *qstring = qerror_human(qerror);
-    qemu_error("%s\n", qstring_get_str(qstring));
+    error_report("%s", qstring_get_str(qstring));
     QDECREF(qstring);
 }