Commit 5aee81a0 introduced a new test for disk-serial. The test fails
on i686 arch because there is no need to add "-cpu qemu32" to command
line. To fix the test update emulator in XML to "/usr/bin/qemu" so we
don't add the "-cpu qemu32" to command while running the test on i686 or
x86_64 host.
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>