Description of problem:

/dev/sdc is a USB key, and I want to have virt-builder write an
OS to this device.  However it doesn't work:

$ virt-builder fedora-20 -o /dev/sdc 
[   1.0] Downloading: http://libguestfs.org/download/builder/fedora-20.xz
#######################################################################   99.9%
[  90.0] Planning how to build this image
[  90.0] Uncompressing
[ 101.0] Resizing (using virt-resize) to expand the disk to 14.9G
virt-builder: libguestfs error: refusing to overwrite block device '/dev/sdc'

(The error comes from the disk-create API.   We should not
be using this API)

This is actually a regression, since this is supposed to work,
and I'm pretty sure it worked before.

Version-Release number of selected component (if applicable):

libguestfs 1.27.5

How reproducible:

See above.