Add missing BLKGETSIZE64 stub; add stubs to _oasis

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add #define _GNU_SOURCE to expose O_DIRECT

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
test: check that opening a non-existent file does in fact fail

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