xend: Reduce xenstore transactions when listing domains

xend: Reduce xenstore transactions when listing domains

In summary, this allows a xenstore transaction object to be passed
around the various device controllers, so that they don't have to do
lots of singleton transactions. Transactions have very heavy I/O
impact from xenstored so reducing their number is important.

When running 3 guests, this patch reduces the impact of 'xm list
--long' from 176 transactions, scaling O(n) with guests, to 26
transactions with O(1) scaling.

I have previously attempted to also address the same issue with 'xm
create' but that's much harder since the device front/back handshake
requires that XenD use a number of small transactions. So i've not
changed anything here.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>