vl.c: Don't print errno after failed qemu_chr_new()

The qemu_chr_new() function doesn't set errno on failure, so
don't print strerror(errno) on the error handling path when
dealing with the -serial, -parallel and -virtioconsole arguments.
This avoids nonsensical error messages like:
  $ ./arm-softmmu/qemu-system-arm -serial wombat
  qemu: could not open serial device 'wombat': Success

We also rephrase the message slightly to make it a little clearer
that we're expecting the name of a QEMU chr backend rather than
a host or guest serial/parallel/etc device.
Reported-by: NChristian Müller <christian.mueller@heig-vd.ch>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

vl.c

@@ -1984,8 +1984,8 @@ static int serial_parse(const char *devname)
     snprintf(label, sizeof(label), "serial%d", index);
     serial_hds[index] = qemu_chr_new(label, devname, NULL);
     if (!serial_hds[index]) {
-        fprintf(stderr, "qemu: could not open serial device '%s': %s\n",
-                devname, strerror(errno));
+        fprintf(stderr, "qemu: could not connect serial device"
+                " to character backend '%s'\n", devname);
         return -1;
     }
     index++;
@@ -2006,8 +2006,8 @@ static int parallel_parse(const char *devname)
     snprintf(label, sizeof(label), "parallel%d", index);
     parallel_hds[index] = qemu_chr_new(label, devname, NULL);
     if (!parallel_hds[index]) {
-        fprintf(stderr, "qemu: could not open parallel device '%s': %s\n",
-                devname, strerror(errno));
+        fprintf(stderr, "qemu: could not connect parallel device"
+                " to character backend '%s'\n", devname);
         return -1;
     }
     index++;
@@ -2041,8 +2041,8 @@ static int virtcon_parse(const char *devname)
     snprintf(label, sizeof(label), "virtcon%d", index);
     virtcon_hds[index] = qemu_chr_new(label, devname, NULL);
     if (!virtcon_hds[index]) {
-        fprintf(stderr, "qemu: could not open virtio console '%s': %s\n",
-                devname, strerror(errno));
+        fprintf(stderr, "qemu: could not connect virtio console"
+                " to character backend '%s'\n", devname);
         return -1;
     }
     qemu_opt_set(dev_opts, "chardev", label);