[Libguestfs] guestfs_list_filesystems: skip block devices which cannot hold file system
Initial discussion is here:
https://www.redhat.com/archives/libguestfs/2018-January/msg00188.html.
v2 was posted here:
https://www.redhat.com/archives/libguestfs/2018-January/msg00246.html.
v3 comparing to v2 is just a rebase with slightly changed commits comments.