PR-1223: Changes to VDI.open_database
PR-1223: Changes to VDI.open_database

- Use constants from Xapi_globs
- Allow VDIs of type redo_log for now
- Attach VDIs with Static_vdis.permanent_vdi_attach
- Detach VDIs after reading metadata

Signed-off-by: John Else <37b74501daa612e48fea94f9846ed4ddeb6a8f07@citrix.com>

djs55/squeezed,koushikcgit/xcp-rrdd,johnelse/xcp-rrdd,sharady/xcp-networkd,simonjbeaumont/xcp-rrdd,djs55/xcp-rrdd,robhoes/squeezed,simonjbeaumont/xcp-rrdd,koushikcgit/xcp-networkd,johnelse/xcp-rrdd,koushikcgit/xcp-rrdd,koushikcgit/xcp-networkd,sharady/xcp-networkd,djs55/xcp-rrdd,djs55/xcp-networkd,djs55/xcp-networkd,koushikcgit/xcp-rrdd