Confuse error message in virtio_init_region_cache()

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.


This is an automated cleanup. This bug report has been moved to QEMU's
new bug tracker on gitlab.com and thus gets marked as 'invalid' now.
Please continue with the discussion here:

 https://gitlab.com/qemu-project/qemu/-/issues/230