qemu.tests: Do system_reset during guest boot up

Add the guest boot up measure functions to help system_reset
case can make sure reset the guest during the guest boot up.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Acked-by: Feng Yang <fyang@redhat.com>
qemu.tests.system_reset_bootable: Start vm in this test explicitly

The vm.verify_alive() function contains some steps to verify the
guest is alive, these steps will cost extra time, and it could
make the boottime checking doesn't exactly match the real situation.

Hence disable the vm starting step in framework and just try to
wait_for_login after starting the vm.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Acked-by: Yiqiao Pu <ypu@redhat.com>
qemu.tests.system_reset_bootable: Update test steps info

Changes from v1:
 * Assign a meaningful name to the '_' variant in for loop.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Acked-by: Yiqiao Pu <ypu@redhat.com>