oxenstored: handling of domain conflict-credit

oxenstored: handling of domain conflict-credit

This commit gives each domain a conflict-credit variable, which will
later be used for limiting how often a domain can cause other domain's
transaction-commits to fail.

This commit also provides functions and data for manipulating domains
and their conflict-credit, and checking whether they have credit.

Reported-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>
Reviewed-by: Jonathan Davies <jonathan.davies@citrix.com>
Reviewed-by: Christian Lindig <christian.lindig@citrix.com>