Fix rebuild with volumes attached

This patch assumes that the correct behavior for instance rebuild is to
maintain attached volumes across a rebuild operation.

Two important changes are:

1) Detaching all volumes during a rebuild so that they won't be 'in-use'
   when prep_block_devices is called to reattach them.

2) xenapi: Allowing additional volumes, not just root volumes, to be
   attached before boot. To handle this, we cycle through all
   block-device-mappings, not just the root-device, create the VDI, and
   later, create the VBD.

Small changes include:

* Using `connection_data` instead of `dev_params` (to match other parts of
  the code base)

* Renaming `get_vdis_for_boot_from_vol` to `get_vdi_uuid_for_volume` to
  reflect its more general and simpler semantics.

Fixes bug 1071547

Change-Id: Ie54a16be4bae2a718ed7d506f32777d0847b9089
Merge "Fix rebuild with volumes attached"
Merge remote-tracking branch 'citrix/master' into build-temp