[Libguestfs] guestfs_list_filesystems: skip block devices which cannot hold file system

[Mykola Ivanets]

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.