VMware: improve log message for attachment of CDROM

Commit 3c59d998c5e6b80549053b01f919ac318cd31b13 config drive
support. The log message makes use of the instance_name which is
not unique. The may be confusing when debugging.

TrivialFix

Change-Id: Iff7f4f938f4980f0c0bf2dc3017ed741453a69f5
Merge "VMware: improve log message for attachment of CDROM"