libxl: fix memory corruption introduced by commit b55cc5f4e

libxl: fix memory corruption introduced by commit b55cc5f4e

Commit b55cc5f4e did a shallow copy of libxl_{sdl,vnc}_info from the
domain config to the build info, which resulted in double-freeing
strings contained in the structures during cleanup, which later
resulted in a libvirtd crash.  Fix by performing a deep copy of the
structure, VIR_STRDUP'ing embedded strings instead of simply copying
their pointers.

Fixes the following issue reported on the libvirt dev list

https://www.redhat.com/archives/libvir-list/2014-August/msg01112.html