Recent work to improve support for loadable driver modules introduced
a regression in the xen driver.  The legacy xen driver is now a
stateful, libvirtd driver but was not being registered when building
without driver modules.

A slight behavior change was also noted in the xen drivers when
built as driver modules.  Previously, explicitly specifying a
connection URI was not necessary, but now

Compiled against library: libvirt 0.10.0
Using library: libvirt 0.10.0
Using API: QEMU 0.10.0
error: failed to get the hypervisor version
error: internal error Cannot find suitable emulator for x86_64

The xen drivers need to be registered before the qemu driver since
the qemu driver will return success with a null connection URI.
This ordering is safe since the xen drivers will decline when not
running the xen kernel.