CA-101777: Relax the checking of mac seed confliction

The confliction between VM mac seeds has been overly restricted, in particular on VM <-> snapshot confliction.  E.g. importing a VM onto a host with its own snapshot leftover is forbidden for no good reason. This changeset trys to relax the checking a little bit.

Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>
CA-101777: Enable dry-run feature for vm-import

which should at least work for metadata only import.

Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>
CA-101777: Let xe-restore-metadata make use of vm-import's dry-run facility

Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>
CA-101777: Add a force flag to xe-restore-metadata (default to false)

Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>
CA-101777: add some comments on the compatability checking logic

Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>
Merge pull request #1312 from zli/xenserver/CA-101777

CA-101777: some improvements on metadata backup/restore