daemon: btrfs: add helper functions mount and umount

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
New API: btrfs_subvolume_get_default

btrfs_subvolume_get_default is for getting the default subvolume of
a btrfs filesystem.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
New API: btrfs_subvolume_show

btrfs_subvolume_show shows the detailed information of a subvolume or
snapshot.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
New API: btrfs_quota_enable

btrfs_quota_enable enables or disables quota for btrfs filesystems.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
New API: btrfs_quota_rescan

btrfs_quota_rescan trashs all qgroup numbers and scans the metadata
again with the current config.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
New API: btrfs_qgroup_limit

btrfs_qgroup_limit limits the size of a qgroup.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
New API: btrfs_qgroup_create

btrfs_qgroup_create creates a new qgroup.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
New API: btrfs_qgroup_destroy

btrfs_qgroup_destroy destroys a qgroup.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
New API: btrfs_qgroup_show

btrfs_qgroup_show shows all qgroups on a btrfs filesystem.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
New API: btrfs_qgroup_assign

btrfs_qgroup_assign adds a qgroup to a parent qgroup.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
New API: btrfs_qgroup_remove

btrfs_qgroup_remove removes a qgroup from its parent qgroup.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>