qemu.test.ipi_x2apic: Typo fix str -> check_string.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.tests.multi_nic_stress: Typo fix % -> %s

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #1057 from FengYang/ipi_x2apic

qemu.test.ipi_x2apic: Typo fix str -> check_string.