exec, memory: Call to xen_modified_memory.

exec, memory: Call to xen_modified_memory.

This patch add some calls to xen_modified_memory to notify Xen about dirtybits
during migration.

Backport of e226939de5814527a21396903b08c3d0ed989558

Note a call to xen_modify_memory has been added to qemu_ram_alloc_from_ptr
as the upstream version does:
  cpu_physical_memory_set_dirty_range(new_block->offset, size, 0xff);
and this function does not exist in 4.2.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Alex Bligh <alex@alex.org.uk>