lxd/storage/drivers/btrfs: Modify received UUID
lxd/storage/drivers/btrfs: Modify received UUID

This adds a new function setReceivedUUID which modifies the "Received
UUID" field on a given subvolume.

This functionality is needed due to the "Received UUID" information
being lost when making a received subvolume read-write.

Signed-off-by: Thomas Hipp <5f82c492b3b00e427412d216ce820707a10c51ce@canonical.com>

lxc/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd