xen: move xenstore devices

Move xenstore related devices (xenstore.c and xenstore_dev.c) from
xen/xenstore to dev/xen/xenstore. This is just code motion, no
functional changes.

Sponsored by: Citrix Systems R&D
Explicitly return None for negative event indices.  Prior to this,
eventat(-1) would return the next-to-last event causing the back button
to cycle back to the end of an event source instead of stopping at the
start.