The error message introduced in commit e45da653223 in
virtio_init_region_cache() was unclear:

    qemu-system-i386: Cannot map used

This patch modifies virtio_error() to prepend all error messages with
"virtio: ", making it easier to trace their origin.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/230
Buglink: https://bugs.launchpad.net/qemu/+bug/1919021

Signed-off-by: Alessandro Ratti <alessandro@0x65c.net>
---
 hw/virtio/virtio.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 9a81ad912e..c43ac4d320 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -3934,11 +3934,16 @@ void virtio_device_set_child_bus_name(VirtIODevice *vdev, char *bus_name)
 void G_GNUC_PRINTF(2, 3) virtio_error(VirtIODevice *vdev, const char *fmt, ...)
 {
     va_list ap;
+    char *prefixed_fmt;
+
+    prefixed_fmt = g_strdup_printf("virtio: %s", fmt);
 
     va_start(ap, fmt);
-    error_vreport(fmt, ap);
+    error_vreport(prefixed_fmt, ap);
     va_end(ap);
 
+    g_free(prefixed_fmt);
+
     if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
         vdev->status = vdev->status | VIRTIO_CONFIG_S_NEEDS_RESET;
         virtio_notify_config(vdev);
-- 
2.39.5