Clean VMX step should always remove floppy.

This PR fixes several issues, all related to trying to use the VMX builder and vpshere post processor.

1. GH issue #1508 - VMX file changes not saving to disk
2. VMX builder won't remove mounted floppies or CD-ROMs

When using the VMX builder its possible for the source machine to have a floppy/CD-ROM configured which gets cloned to the new VM Packer spins up. The new VM's Packer config doesn't have a floppy_files/iso_path config entry, the Packer clean VMX step fails to remove the mounted media from the new VM.

This can cause build failures, for example with the vsphere post processor generating errors like:
```
* Post-processor failed: Failed: exit status 1
Error: File (/home/teamcity/tmp/buildTmp/packer941120499) could not be found.
```