xenstore: remove the suspend sx lock

There's no need to prevent suspend while doing xenstore transactions,
callers of transactions are supposed to be prepared for a transaction
to fail.

This fixes a bug that could be triggered from the xenstore user-space
device, since starting a transaction from user-space would result in
returning there with a sx lock held, that causes a WITNESS check to
trigger.

Tested by: Nathan Friess <nathan.friess@gmail.com>
Sponsored by: Citrix Systems R&D