Description of problem:

$ guestfish -N part mkswap /dev/sda1 : pvcreate /dev/sda1
libguestfs: error: pvcreate: WARNING: swap signature detected on /dev/sda1. Wipe it? [y/n] 
  Aborting pvcreate on /dev/sda1.

But note that other content is fine.  For example if the
partition contained an ext2 filesystem before, then pvcreate works:

$ guestfish -N part mkfs ext2 /dev/sda1 : pvcreate /dev/sda1
[no error]

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

libguestfs 1.20.0

How reproducible:

100%

Steps to Reproduce:
See above.

Additional information:

For some reason this especially affects the test suite under
RHEL 7.  It may be that a combination of tests has been disabled,
such that when we come to test pvcreate there happens to be
a swap space signature on the partition.