virttest.virsh: Add virsh migrate setspeed/getspeed functions

migrate_setspeed(domain, bandwidth, extra=None, **dargs)
migrate_getspeed(domain, **dargs)
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.
Merge pull request #1056 from Guannan-Ren/master

virttest.virsh: Add virsh migrate setspeed/getspeed functions