tools/libxl: Fix free() of wild pointer in libxl__initiate_device_remove()

tools/libxl: Fix free() of wild pointer in libxl__initiate_device_remove()

libxl__initiate_device_remove() had a preexisting error path issue where
libxl_dominfo_dispose() could be called on a libxl_dominfo object before it
had been initialised with libxl_dominfo_init().

This was safe until c/s ab44401 added the pointer ssid_label, which point
libxl_dominfo_dispose() free()s.

Unconditionally initialise info in libxl__initiate_device_remove() before
taking an error path which will free it.

Coverity-ID: 1223212
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>