libxenlight: fix segfault when domid_to_name returns NULL

libxenlight: fix segfault when domid_to_name returns NULL

The function libxl_domid_to_name() can return NULL if the path
/local/domain/%d/name does not exist.  This causes a segfault if the
NULL name is later passed as a value to libxl_xs_writev().  I'm
hitting this making a call to libxl_device_vfb_add() from my graphical
switcher application.

This patch modifies xs_writev() and libxl_xs_writev() to skip NULL
values.

Signed-off-by: Eamon Walsh <ewalsh@tycho.nsa.gov>