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/hw/virtio-net.c b/hw/virtio-net.c
index 8359be6..be33c68 100644
--- a/hw/virtio-net.c
+++ b/hw/virtio-net.c
@@ -765,7 +765,7 @@
 
     if (version_id >= 7) {
         if (qemu_get_be32(f) && !peer_has_vnet_hdr(n)) {
-            qemu_error("virtio-net: saved image requires vnet_hdr=on\n");
+            error_report("virtio-net: saved image requires vnet_hdr=on");
             return -1;
         }
 
@@ -794,7 +794,7 @@
 
     if (version_id >= 11) {
         if (qemu_get_byte(f) && !peer_has_ufo(n)) {
-            qemu_error("virtio-net: saved image requires TUN_F_UFO support\n");
+            error_report("virtio-net: saved image requires TUN_F_UFO support");
             return -1;
         }
     }