Re-add OASIS build infrastructure

This will allow us to more easily conditionally compile different
xenopsd backends.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Remove some files which were duplicated from the lib/ directory

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add C stubs missing from the libxl backend

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