hw/xen: Implement XenStore transactions

hw/xen: Implement XenStore transactions

Given that the whole thing supported copy on write from the beginning,
transactions end up being fairly simple. On starting a transaction, just
take a ref of the existing root; swap it back in on a successful commit.

The main tree has a transaction ID too, and we keep a record of the last
transaction ID given out. if the main tree is ever modified when it isn't
the latest, it gets a new transaction ID.

A commit can only succeed if the main tree hasn't moved on since it was
forked. Strictly speaking, the XenStore protocol allows a transaction to
succeed as long as nothing *it* read or wrote has changed in the interim,
but no implementations do that; *any* change is sufficient to abort a
transaction.

This does not yet fire watches on the changed nodes on a commit. That bit
is more fun and will come in a follow-on commit.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>