dm_op: convert HVMOP_modified_memory

dm_op: convert HVMOP_modified_memory

This patch introduces code to handle DMOP continuations.

NOTE: This patch also modifies the type of the 'nr' argument of
      xc_hvm_modified_memory() from uint64_t to uint32_t. In practice the
      value passed was always truncated to 32 bits.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>