Comparing device name values instead of their reference

Although end users specified device name as `vda' but since `is' in
python is used to compare reference, bootable volume is not
detected. We should use `==' to compare values instead.

Fixes bug #1229610

Change-Id: I7f127dc489706482f6bb245372bfd8079b91c40f
Merge "Comparing device name values instead of their reference"