vmware: fix parse of disk source

vmware: fix parse of disk source

While writing disk source refactoring, I discovered that conversion
from XML to vmware modified the disk source in place; if the same
code is reached twice, the second call behaves differently because
the first call didn't clean up its mess.

* src/vmware/vmware_conf.c (vmwareParsePath): Avoid munging input
string.
* src/vmware/vmware_conf.h (vmwareParsePath): Make static.

Signed-off-by: Eric Blake <eblake@redhat.com>