Confuse error message in virtio_init_region_cache()

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1919021
Reported by 'Philippe Mathieu-Daudé' on 2021-03-13 :

The error message added in commit e45da653223 to
virtio_init_region_cache()
are somehow confuse:

  qemu-system-i386: Cannot map used

It would be helpful to more explicit string, including "virtio" prefix.

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