Fix for volume detach error when use NFS as the cinder backend

For NFS volumes, every time you detach a volume, nova tries to umount
the device path. If the device path is busy (or in use), it should
output a message to log and continue.
In current code, if the device is busy, it cannot catch the ‘device is busy’
message returned by umount, because it looking for the ‘target is busy’.
 Therefore, current code skip the ‘if’ statement and run the ‘else’ and
raise the exception.
Fix: Add ‘device is busy’ to if statement.

Add a mock test to check the behaviour of the
virt.libvirt.volume.LibvirtNFSVolumeDriver.disconnect_volume
when it has umount errors.

Closes-Bug: #1340552

Change-Id: Iac946c37064c5f5bf5a102305de40d21d16846c1
Merge "Fix for volume detach error when use NFS as the cinder backend"