vhost: handle dirty pages logging request

vhost: handle dirty pages logging request

VHOST_USER_SET_LOG_BASE request is used to tell the backend (dpdk
vhost-user) where we should log dirty pages, and how big the log
buffer is.

This request introduces a new payload:

    typedef struct VhostUserLog {
            uint64_t mmap_size;
            uint64_t mmap_offset;
    } VhostUserLog;

Also, a fd is delivered from QEMU by ancillary data.

With those info given, an area of memory is mmaped, assigned
to dev->log_base, for logging dirty pages.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: Victor Kaplansky <victork@redhat.com>
Tested-by: Pavel Fedin <p.fedin@samsung.com>