Merge pull request #3 from aaannz/pvscsi

Xen4.2 ABI compat
pvscsi: update comments about libxl.so ABI

Signed-off-by: Olaf Hering <olaf@aepfle.de>
pvscsi: fix double free in scsi-attach

Signed-off-by: Olaf Hering <olaf@aepfle.de>
pvscsi: avoid double assignment of host devices

Signed-off-by: Olaf Hering <olaf@aepfle.de>
pvscsi: check null pointer in libxl__add_vscsis

Signed-off-by: Olaf Hering <olaf@aepfle.de>
define SUSE PVSCSI extension for 3rd party use