xenapi: make auto_config_disk persist boot flag

Currently auto_config_disk creates a new partition that ignores many
settings on the original partition.

The key flag that should be respected is the "boot" flag. When present
on the old partition, it should be present on the new partition.

Change-Id: I0408112cd60046422d6863651323fc9a7313d488
Closes-bug: #1265465
Merge "xenapi: make auto_config_disk persist boot flag"