revert QEMU q35 platform use from r346748

It seems to be incompatible with the OVMF.fd (of unknown provenance)
in use by the Cirrus-CI config.  We will soon have a known OVMF build
via a port/package (see review D19869) and we can switch back to q35
once packages are available.

Discussed with:	bcran

tools/boot/ci-qemu-test.sh

@@ -97,7 +97,7 @@ trap tempdir_cleanup EXIT SIGINT SIGHUP SIGTERM SIGQUIT
 # And, boot in QEMU.
 : ${BOOTLOG:=${TMPDIR:-/tmp}/ci-qemu-test-boot.log}
 timeout 300 \
-    qemu-system-x86_64 -M q35 -m 256M -nodefaults \
+    qemu-system-x86_64 -m 256M -nodefaults \
    	-drive if=pflash,format=raw,readonly,file=${OVMF} \
         -serial stdio -vga none -nographic -monitor none \
         -snapshot -hda fat:${ROOTDIR} 2>&1 | tee ${BOOTLOG}