mount: Add mount_vfs_nochroot

This internal function allows mounting a mountable outside /sysroot.
btrfs: Update btrfs_subvolume_list to take Mountable_or_Path

btrfs_subvolume_list can now take either the path of a mounted btrfs
filesystem, or a mountable describing the location of a btrfs
filesystem, or one of its volumes. In the latter case, the filesystem
will be automatically mounted outside of /sysroot before running the
btrfs tool, and unmounted afterwards.