libxc: skip a pointless madvise() call

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
xl: remove file lock

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
libxenstore: throw away error responses to allow write pipelining

In particular this should speed up code which does
  write k = v
  write k = v
  ...
  write k = v
because all the 'write k' requests will be sent without waiting for
the 'unit' results to come back.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>