tests: fix qemuxml2argvtest to be arch independent

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: NPavel Hrdina <phrdina@redhat.com>

tests/qemuxml2argvdata/qemuxml2argv-disk-serial.args

 LC_ALL=C PATH=/bin HOME=/home/test USER=test LOGNAME=test QEMU_AUDIO_DRV=none \
-/usr/libexec/qemu-kvm -S -M pc -cpu qemu32 -m 214 -smp 1 -nographic -nodefaults \
+/usr/bin/qemu -S -M pc -m 214 -smp 1 -nographic -nodefaults \
 -monitor unix:/tmp/test-monitor,server,nowait -no-acpi -boot c -usb \
 -drive 'file=/dev/HostVG/QEMUGuest1,if=none,\
 id=drive-ide0-0-1,serial=\ \ WD-WMAP9A966149' \

tests/qemuxml2argvdata/qemuxml2argv-disk-serial.xml

@@ -13,7 +13,7 @@
   <on_reboot>restart</on_reboot>
   <on_crash>destroy</on_crash>
   <devices>
-    <emulator>/usr/libexec/qemu-kvm</emulator>
+    <emulator>/usr/bin/qemu</emulator>
     <disk type='block' device='disk'>
       <source dev='/dev/HostVG/QEMUGuest1'/>
       <target dev='hda' bus='ide'/>