The command "info migrate" of qemu outputs the dirty-pages-rate during
migration, but page size is different in different architectures. So
page size should be output to calculate dirty pages in bytes.

Page size is already implemented with commit
030ce1f8612215fcbe9d353dfeaeb2937f8e3f94 in qemu.
Now Implement the counter-part in libvirt.
Signed-off-by: NChao Fan <fanc.fnst@cn.fujitsu.com>
Signed-off-by: NLi Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>