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/qemu-config.c b/qemu-config.c
index 2c9a7a5..8e06770 100644
--- a/qemu-config.c
+++ b/qemu-config.c
@@ -313,7 +313,7 @@
             break;
     }
     if (lists[i] == NULL) {
-        qemu_error("there is no option group \"%s\"\n", group);
+        error_report("there is no option group \"%s\"", group);
     }
     return lists[i];
 }
@@ -327,7 +327,7 @@
 
     rc = sscanf(str, "%63[^.].%63[^.].%63[^=]%n", group, id, arg, &offset);
     if (rc < 3 || str[offset] != '=') {
-        qemu_error("can't parse: \"%s\"\n", str);
+        error_report("can't parse: \"%s\"", str);
         return -1;
     }
 
@@ -338,8 +338,8 @@
 
     opts = qemu_opts_find(list, id);
     if (!opts) {
-        qemu_error("there is no %s \"%s\" defined\n",
-                   list->name, id);
+        error_report("there is no %s \"%s\" defined",
+                     list->name, id);
         return -1;
     }
 
@@ -357,7 +357,7 @@
 
     rc = sscanf(str, "%63[^.].%63[^=]%n", driver, property, &offset);
     if (rc < 2 || str[offset] != '=') {
-        qemu_error("can't parse: \"%s\"\n", str);
+        error_report("can't parse: \"%s\"", str);
         return -1;
     }