domain/xl: import vcpuaffinity macro from libvirt

This patch includes a macro from libvirt to fix issues with libxl_set_vcpuaffinity() that arise with newer xen versions. Beginning with version 4.5 the functions requires an additional parameter for 'VCPU soft affinity' that cosmos does not use anyway.

The macro is taken from linvirt. See https://gitlab.com/libvirt/libvirt/commit/bfc72e99920215c9b004a5380ca61fe6ff81ea6b for more details.