xenstored: allow guest to shutdown all its watches/transactions

xenstored: allow guest to shutdown all its watches/transactions

During kexec all old watches have to be removed, otherwise the new
kernel will receive unexpected events. Allow a guest to reset itself
and cleanup all of its watches and transactions.

Add a new XS_RESET_WATCHES command to do the reset on behalf of the
guest.

(Changes by iwj: specify the argument to be a single nul byte.  Permit
read-only clients to use the new command.)

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>