[Libguestfs] Supporting btrfs subvolumes during inspection

[Matthew Booth]

We've currently got a bug in libguestfs which means we can't inspect 
filesystems in btrfs subvolumes:

https://bugzilla.redhat.com/show_bug.cgi?id=824021

This is the default configuration if you select btrfs in F17+. The issue 
is that it requires an api to fix it, as the return values of 
inspect_os, inspect_get_filesystems and inspect_get_mountpoints can't 
express a btrfs subvolume as they're expected to be the names of block 
devices.

As a starter for 10, I propose the addition of parallel apis suffixed 
_ext which return annotated device descriptions, e.g.:

block:/dev/vda2
btrfsvol:/dev/vda4,root

The string before the colon is a descriptor. The format of the string 
after the colon is custom to the descriptor, and is described in the 
API. A client program can ignore strings with unknown descriptors. New 
descriptors can be added as required.

The old apis would return the same as the _ext versions, except they 
would only return block devices. Other devices would be omitted.

Matt
-- 
Matthew Booth, RHCA, RHCSS
Red Hat Engineering, Virtualisation Team

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