xenapi: refactor get_all_vdis_in_sr

Making the code in vm_utils._get_all_vdis_in_sr more consistent with the
code in session.get_all_refs_and_recs.

Rather than catching all exceptions, the session.get_rec explicitly
catches the INVALID_HANDLE exception that xenapi might raise.

Change-Id: Ic69735e1a6caa893f63428141428ee0fd1a3ac65
Merge "xenapi: refactor get_all_vdis_in_sr"