qga: add guest-get-diskstats command for Linux guests

Add a new 'guest-get-diskstats' command for report disk io statistics
for Linux guests. This can be useful for getting io flow or handling
IO fault, no need to enter guests.

Signed-off-by: luzhipeng <luzhipeng@cestc.cn>
Message-Id: <20220520021935.676-1-luzhipeng@cestc.cn>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>