Make it possible to run valgrind on code linked with the libxc libraries.

Make it possible to run valgrind on code linked with the libxc libraries.
Valgrind cannot see when a hypercall has initialised entries in a data
structure, so appropriate memsets have been placed before using dom0_op_t,
privcmd_hypercall_t, and a couple of miscellaneous blocks passed into this
layer.  This initialises the block so that valgrind considers it to be valid,
but the data therein will be immediately overwritten by the hypercall, all
being well.

These changes are semantically neutral if -DVALGRIND is not set.

Signed-off-by: Ewan Mellor <ewan@xensource.com>