Makefile: Install qemu-img and qemu-nbd man pages only if built

Makefile: Install qemu-img and qemu-nbd man pages only if built

When splitting openSUSE's qemu and qemu-linux-user packages we noticed
that for linux-user-only builds unrelated man pages got installed.
It's surely possible to delete them before packaging, but not installing
them in the first place seems more logical.

Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>