virttest.qemu_vm: VM.verify_alive() changes

Hi guys,

the first patch adds support for catching KVM internal errors (They are easily to seen by executing `qemu_no_shutdown` https://github.com/autotest/virt-test/pull/379 test when using `reset_interval = 0`, using smp).

The second patch is only a proposal how to speed-up another similar function.

Regards,
Lukáš