lxd/device/disk: Fallback to using mount device path for major/minor number extraction for BTRFS
lxd/device/disk: Fallback to using mount device path for major/minor number extraction for BTRFS

If `btrfs filesystem show` doesn't work then fallback to extracting major/minor number directly from the parent block device in the mount info for cgroup limits generation.

Fixes #8564

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

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