Coverity: Fix the forward_null error in Python binding codes

Coverity: Fix the forward_null error in Python binding codes

Related coverity log:

Error: FORWARD_NULL:
/builddir/build/BUILD/libvirt-0.9.10/python/libvirt-override.c:355:
assign_zero: Assigning: "params" = 0.
/builddir/build/BUILD/libvirt-0.9.10/python/libvirt-override.c:458:
var_deref_model: Passing null variable "params" to function
"getPyVirTypedParameter", which dereferences it. (The dereference is assumed on
the basis of the 'nonnull' parameter attribute.)