Fix VirtualInterfaceMacAddressException message

Previous the message reflected the default value of
create_unique_mac_address_attempts configuration option.

Changed to a generic message.

Change-Id: I55c5f95cfc7ac4183e08a7880cbbb9670a02cef4
Closes-Bug: #1276312
Stacktrace on error from libvirt during unfilter

We are seeing this issue freqently in the gate, but we don't have any
stacktrace for what is happening.  An error from libvirt is an
unexpected exception and should include a stacktrace.

Related-Bug: #1275256
Change-Id: If7c0e19aba7a3d129f4b82f52d86b8e3b6a0a194
Merge "Stacktrace on error from libvirt during unfilter"
Merge "Fix VirtualInterfaceMacAddressException message"