Description of problem:

(Reported by Исаев Виталий Анатольевич)

When a PV is missing, pvs should not return the literal string
"unknown device".

For example:

$ guestfish -N disk -N disk pvcreate /dev/sda : pvcreate /dev/sdb : vgcreate VG "/dev/sda /dev/sdb"
$ guestfish -a test1.img run : pvs
/dev/sda
unknown device                       << note

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

libguestfs 1.16 - 1.25

How reproducible:

100%

Steps to Reproduce:
1. See reproducer above.

Expected results:

I'm not sure what it should do here.  Probably it should ignore
the missing device.  An alternative is to print an error.