On one occasion, hdev_open() returned -1 in case of an unknown error
instead of a proper -errno value. Adjust this to match the behavior of
raw_open() (in raw-win32), which is to return -EINVAL in this case.
Also, change the call to error_setg*() to match the one in raw_open() as
well.
Signed-off-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>