On AWS, we should not reuse device names as long as possible, see
https://aws.amazon.com/premiumsupport/knowledge-center/ebs-stuck-attaching/

"If you specify a device name that is not in use by EC2, but is being used by
the block device driver within the EC2 instance, the attachment of the EBS
volume does not succeed and the EBS volume is stuck in the attaching state."

This patch adds a device name allocator that tries to find a name that's next
to the last used device name instead of using the first available one.
This way we will loop through all device names ("xvdba" .. "xvdzz") before
a device name is reused.