Fix message handling with VirtualInterfacePlugException class usage
Fix message handling with VirtualInterfacePlugException class usage

When a VNIC attach fails, the actual error message thrown from the
platform gets swallowed in the console showing only the generic
message in the exception class. This is due to the formatting
string passed to the exception class, though the
VirtualInterfacePlugException class definition doesn't accept one.

Change-Id: I18be7fc2284387d2f4d492ca68de6091f4bb8b66
Closes-Bug: #1796072

openstack/nova-powervm,stackforge/nova-powervm,stackforge/nova-powervm,openstack/nova-powervm