configure: fix handling of --docdir parameter

configure: fix handling of --docdir parameter

Commit ca8c0909f01 changed qemu_docdir to be docdir, then later uses the
qemu_docdir name in the final assignment. Unfortunately, one instance of
qemu_docdir was missed: the one which comes from the --docdir parameter.
This patch restores the proper handling of the --docdir parameter.

Fixes: ca8c0909f01 ("configure: build docdir like other suffixed
directories")

Signed-off-by: Bruce Rogers <brogers@suse.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201015190742.270629-1-brogers@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>