libxl: introduce virtxend daemon

libxl: introduce virtxend daemon

The virtxend daemon will be responsible for providing the libxl API
driver functionality. The libxl driver is still loaded by the main
libvirtd daemon at this stage, so virtxend must not be running at
the same time.

This naming is slightly different than other drivers. With the libxl
driver, the user still has a 'xen:///system' URI, and we provide it
in a libvirt-daemon-xen RPM, which pulls in a
libvirt-daemon-driver-libxl RPM.

Arguably we could rename the libxl driver to "xen" since it is the
only xen driver we have these days, and that matches how we expose it
to users in the URI naming.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>