Fix attaching config drive issue on Hyper-V when migrate instances

After instance resized or migrated on Hyper-V hypervisor.
The configdrive iso or vhd is copied to resized or migrated
instance, but is not attached to instance.

Because there are configurations for config drive like
config_drive_cdrom, config_drive_format, and the configurations
on different Hyper-V compute node may be different. it will need
to convert configdrive format after resized or migrated.
It is easy to convert from iso9660 or vfat to vhd, but it seems
impossible to convert from vhd to iso9660 or vfat.
So this commit just ignore the target Hyper-V compute node's
config drive configurations, leave the original config drive format.

Change-Id: I349e3b2221fff0ae217a71a91895afd21ff7d18d
Closes-Bug: #1321640

Conflicts:
	nova/virt/hyperv/pathutils.py