tests/qtest/virtio-iommu-test: Don't pass uninitialized data into qtest_memwrite

tests/qtest/virtio-iommu-test: Don't pass uninitialized data into qtest_memwrite

Valgrind complains about:

Use of uninitialised value of size 8
&
Conditional jump or move depends on uninitialised value(s)

both at:
   at 0x5265931: _itoa_word (_itoa.c:180)
   by 0x527EEC7: __vfprintf_internal (vfprintf-internal.c:1687)
   by 0x528C8B0: __vsprintf_internal (iovsprintf.c:96)
   by 0x526B920: sprintf (sprintf.c:30)
   by 0x1296C7: qtest_memwrite (libqtest.c:1273)
   by 0x193C04: send_map (virtio-iommu-test.c:125)
   by 0x194392: test_attach_detach (virtio-iommu-test.c:214)
   by 0x17BDE7: run_one_test (qos-test.c:181)
   by 0x4B0699D: test_case_run (gtestutils.c:2900)
   by 0x4B0699D: g_test_run_suite_internal (gtestutils.c:2988)
   by 0x4B068B2: g_test_run_suite_internal (gtestutils.c:3005)
   by 0x4B068B2: g_test_run_suite_internal (gtestutils.c:3005)
   by 0x4B068B2: g_test_run_suite_internal (gtestutils.c:3005)
 Uninitialised value was created by a stack allocation
   at 0x193AFD: send_map (virtio-iommu-test.c:103)

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20241209204427.17763-5-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>