RFC: extend the xenstore ring with a 'closing' signal

Currently hvmloader uses the xenstore ring and then tries to
reset it back to its initial state. This is not part of the
ring protocol and, if xenstored reads the ring while it is
happening, xenstored will conclude it is corrupted. A corrupted
ring will prevent PV drivers from connecting. This seems to
be a rare failure.

Furthermore, when a VM crashes it may jump to a 'crash kernel'
to create a diagnostic dump. Without the ability to safely
reset the ring the PV drivers won't be able to reliably
establish connections, to (for example) stream a memory dump to
disk.

This prototype patch contains a simple extension of the
xenstore ring structure, enough to contain version numbers and
a 'closing' flag. This memory is currently unused within the
4k page and should be zeroed as part of the domain setup
process. The oxenstored server advertises version 1, and
hvmloader detects this and sets the 'closing' flag. The server
then reinitialises the ring, filling it with obviously invalid
data to help debugging (unfortunately blocks of zeroes are
valid xenstore packets) and signals hvmloader by the event
channel that it's safe to boot the guest OS.

Updates since initial version (thanks to Paul Durrant for the
review):
* remove unused client version and associated dead code
* treat 'closing' as a flag by using "!=0" rather than "==1"
* hvmloader: move function up and remove forward decl
* document the existing xenstore ring and the extention in misc/

Signed-off-by: David Scott <dave.scott@citrix.com>