New API: list-filesystems: list filesystems

New API: list-filesystems: list filesystems

This API is a simpler replacement for the guestfish commands
list-devices / list-partitions / lvs, in the case where you are
just examining a guest by hand to see what it contains.

Typical usage and output in guestfish is like this:

$ guestfish --ro -a /dev/vg_trick/F13x64
><fs> run
><fs> list-filesystems
/dev/vda1: ext4
/dev/vg_f13x64/lv_root: ext4
/dev/vg_f13x64/lv_swap: swap

It can also be used to replace programs that try to mount
devices to determine if they are mountable filesystems.