Add a new pvdrivers header to serve as the register of product numbers.

Add a new pvdrivers header to serve as the register of product numbers.

These product numbers are used by the QEMU blacklisting protocol in
traditional QEMU and are currently coded directly into the xenstore.c
source module. Since there are now multiple QEMUs this information
should be pulled into a public header to avoid duplication/conflict.
hvm-emulated-unplug.markdown has also been adjusted to reference the
new header.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Keir Fraser <keir@xen.org>