storage/reflink: use context managers in is_supported()
storage/reflink: use context managers in is_supported()

Don't rely on garbage collection to close and remove the tempfiles.

QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin