Compute and return the footer checksum

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
test: verify footer checksums

This test currently fails.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Marshal the footer uuid as bytes, not as an ASCII string

This fixes the broken unit test.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>