xen: convert the xenstore user-space char device to a newbus device

Convert the xenstore user-space device (/dev/xen/xenstore) to a device
using the newbus interface. This allows us to make the device
initialization dependant on the initialization of xenstore itself in
the kernel.

Sponsored by: Citrix Systems R&D

dev/xen/xenstore/xenstore.c:

• Convert to a newbus device, this removes the xs_dev_init function.

xen/xenstore/xenstore_internal.h:

• Remove xs_dev_init prototype.

dev/xen/xenstore/xenstore.c:

• Don't call xs_dev_init anymore, the device will attach itself when xenstore is started.