storage: btrfs subvolumes for directory pools

This commit adds support for btrfs subvolumes as directory volumes.  The
directory storage pool can be used to manage btrfs subvolumes on an existing
btrfs filesystem.  The driver can create new blank subvolumes and snapshots
of existing subvolumes as well as delete existing subvolumes.

The subvolumes created are automatically made visible on the host side
and can be attached to domains using the <filesystem> tags as defined in
'format domain' documentation.

Subvolumes do not implement quotas at the moment because the current
(btrfs-progs-0.20.rc1.20130501git7854c8b-4.fc20.x86_64) support for quota
management in btrfs-progs is lacking the necessary features, for example
it's not possible to see the quota assigned to a certain subvolume and
usage information is only updated on syncfs(2).  Quota support will be
implemented once the tools gain the necessary features.

Signed-off-by: Oskari Saarenmaa <os@ohmu.fi>