i386/xen: Initialize XenBus and legacy backends from pc_init1()

i386/xen: Initialize XenBus and legacy backends from pc_init1()

Now that we're close to being able to use the PV backends without actual
Xen, move the bus instantiation out from xen_hvm_init_pc() to pc_init1().

However, still only do it for (xen_mode == XEN_ATTACH) (i.e. when running
on true Xen) because we don't have XenStore ops for emulation yet, and
the XenBus instantiation failure is fatal. Once we have a functional
XenStore for emulated mode, this will become (xen_mode != XEN_DISABLED).

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>