Hyper-V:Preserve config drive image after the instance is resized

If we configure config_drive_cdrom=True in nova.conf,
the config drive image will be lost after the instance
is resized on hyperV. This is because nova compute will
not copy the disk file if the file's ResourceSubType
is not 'Virtual Hard Disk'.

This commit adds 'Virtual CD/DVD Disk' into disk_resources
to preserve the config drive image after the instance is
resized.

Closes-Bug: #1250324

Change-Id: Ia125a8b132d194ddc2230754fd1b813698b410a8
Revert "Fix race conditions between imagebackend and imagecache"

This reverts commit 2593469103aa7d9d2bcb759b78d5f8637911a1e0

This is a possible cause for bug #1273556

Change-Id: I675806a21bd2e2ab438913690438b2d25b145f7c
Merge "Revert "Fix race conditions between imagebackend and imagecache""
Merge "Hyper-V:Preserve config drive image after the instance is resized"