Support leases in guest XML and lock manager

Support leases in guest XML and lock manager

A lock manager may operate in various modes. The direct mode of
operation is to obtain locks based on the resources associated
with devices in the XML. The indirect mode is where the app
creating the domain provides explicit leases for each resource
that needs to be locked. This XML extension allows for listing
resources in the XML

  <devices>
     ...
     <lease>
       <lockspace>somearea</lockspace>
       <key>thequickbrownfoxjumpsoverthelazydog</key>
       <target path='/some/lease/path' offset='23432'/>
     </lease>
     ...
  </devices>

The 'lockspace' is a unique identifier for the lockspace which
the lease is associated

The 'key' is a unique identifier for the resource associated
with the lease.

The 'target' is the file on disk where the leases are held.

* docs/schemas/domain.rng: Add lease schema
* src/conf/domain_conf.c, src/conf/domain_conf.h: parsing and
  formatting for leases
* tests/qemuxml2argvdata/qemuxml2argv-lease.args,
  tests/qemuxml2argvdata/qemuxml2argv-lease.xml,
  tests/qemuxml2xmltest.c: Test XML handling for leases