Support multiple StoragePlugin instances provided by a single library

Allows to make available multiple distinct filesystems, for example one
for internal memory of a device and another for removable memory card.
Load FSStoragePlugin configuration from a file.

No more hardcoded values, every aspect of each storage including
blacklisted paths is now set up in /etc/buteo/mtp-fsstorage-conf.xml.
Change StorageID assigning as per Richard Braakman's suggestion

This should perhaps be assignStorageId(1, i + 1). In the commented-out
code that handles multiple plugins, the first argument is the plugin
number. The second one is called partitionNo. I'm not sure it matters
anywhere, but I guess that conceptually the home dir and the sd card are
partitions of the fsstorageplugin.
Move mtp-fsstorage-conf.xml to sample-vendor-configuration package
Use external blacklist

The file has a trivial path-per-line format allowing for its easy
sharing between applications.
Merge pull request #20 from xhaakon/multi-fsstorage

[mtp] Load FSStoragePlugin configuration from a file. Fixes JB#9613