Add ability to force enable/disable of tools build

Add ability to force enable/disable of tools build

The qemu-img, qemu-nbd and qemu-io tools are built conditionally
based on whether any softmmu target is enabled. These are useful
self-contained tools which can be used in many other scenarios.
Add new --enable-tools/--disable-tools args to configure to allow
the user to explicitly turn on / off their build. The default
behaviour is now to build these tools are all times, regardless
of whether any softmmu target is enabled

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>