virttest.qemu_vm: Add check for KVM internal error

Verify KVM internal error occurance during VM.verify_alive().

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
virttest.qemu_vm: Speed-up the verify_userspace_crash()

Make verify_userspace_crash() a bit faster for normal execution (a bit
slower on failure, but the test is already failing so it's not an issue).

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>