Support for btrfs volumes

This patch adds support for creating btrfs volumes.
For example:

<partition fs="btrfs" name="/mnt/testarea" size="10" type="part"/>

will create a btrfs volume of 10G mounted at /mnt/testarea.
Multiple btrfs volumes can be created.

Bug: 973595
Change-Id: Id1c0175e355b550321a626199ea258fcd7489e3f
Merge "Support for btrfs volumes" into release-0.13