Move _is_mapping logic to more central place

[Thang Pham]

This bug is a follow-up to Nikola Dipanov's comment in https://review.openstack.org/#/c/109834/2/nova/compute/manager.py.
The logic to identify volumes is currently a nested function in _default_block_device_names, named _is_mapping. It should be moved to a more general place so others could utilize it.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/114799

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/114799
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=062b1f8c0f6ba09ab6764ea512c3615fc93aaf08
Submitter: Jenkins
Branch: master

commit 062b1f8c0f6ba09ab6764ea512c3615fc93aaf08
Author: Thang Pham <email address hidden>
Date: Sat Aug 16 21:40:29 2014 -0400

    Move _is_mapping to more central location

    The logic to identify volumes is currently a nested function
    in _default_block_device_names, named _is_mapping. It should
    be moved to a more general location so others could utilize it
    and allow it to be properly unit tested. The following patch
    moves _is_mapping to nova/virt/block_device.py and renames it
    to is_block_device_mapping.

    Change-Id: I560abc4b57ca5bd195282af7cd1ab9bbf7600b67
    Closes-Bug: #1351810