qga: fix missing closedir() in qmp_guest_get_disks()

We opendir("/sys/block") at the beginning of the function, but we never
close it prior to returning.

Fixes: Coverity CID 1436130
Fixes: fed39564 ("qga: add implementation of guest-get-disks for Linux")
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Tomáš Golembiovský <tgolembi@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>