Return NULL and set an exception if allocation fails
Return NULL and set an exception if allocation fails

This is a recommended work-flow for allocation failures and we should
follow it.

Signed-off-by: Pavel Hrdina <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>

libvirt/libvirt-python,libvirt/libvirt-python,libvirt/libvirt-python