libxl: colo: make it depend on availability of libnl

libxl: colo: make it depend on availability of libnl

Netlink is required when initialising COLO, so make sure only to compile
COLO only when netlink is available. Change the inclusion of
linux/netlink.h to netlink/netlink.h so that it doesn't use Linux kernel
header directly.

Provide necessary stub functions in case COLO is disabled. This should
fix build on FreeBSD because there is no netlink there. It would also
make libxl build properly when netlink is not present on a Linux
system.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>