tests: avoid dirname in tests

qemuxml2argvtest sanitizes PATH to just /bin, but on at least
Fedora 16, dirname lives in /usr/bin instead.  Regression
introduced in commit e7201afd.

* tests/qemuxml2argvdata/qemu.sh: Avoid forking a dirname call,
since dirname might not be in PATH after test sanitization.
* tests/qemuxml2argvdata/qemu-supported-cpus.sh: Likewise.
Diagnosed by Michal Privoznik.

tests/qemuxml2argvdata/qemu-supported-cpus.sh

 #! /bin/sh
 
-. $(dirname $0)/qemu-lib.sh
+. ${0%/*}/qemu-lib.sh
 
 case $* in
 "-M ?")

tests/qemuxml2argvdata/qemu.sh

 #! /bin/sh
 
-. $(dirname $0)/qemu-lib.sh
+. ${0%/*}/qemu-lib.sh
 
 case $* in
 "-M ?")