qemu.tests.boot_from_device: Change vm.create() to preprocess_vm()

In older code we did not update params for vm before creating it.
So old VM is reused. So we will meet following error:
15:42:02 ERROR| Exception escaping from test:
Traceback (most recent call last):
  File "/usr/code/autotest/client/shared/test.py", line 426, in _exec
    _call_test_function(self.execute, *p_args, **p_dargs)
  File "/usr/code/autotest/client/shared/test.py", line 853, in _call_test_function
    raise error.UnhandledTestFail(e)
UnhandledTestFail: Unhandled VMImageMissingError: CD image file not found: '/tmp/test.iso'
[context: (run_boot_from_device) --> creating 'virt-tests-vm1']
Traceback (most recent call last):
  File "/usr/code/autotest/client/shared/test.py", line 846, in _call_test_function
    return func(*args, **dargs)
  File "/usr/code/autotest/client/shared/test.py", line 299, in execute
    postprocess_profiled_run, args, dargs)
  File "/usr/code/autotest/client/shared/test.py", line 216, in _call_run_once
    *args, **dargs)
  File "/usr/code/autotest/client/shared/test.py", line 322, in run_once_profiling
    self.run_once(*args, **dargs)
  File "/usr/code/autotest/client/tests/virt/virt.py", line 139, in run_once
    run_func(self, params, env)
  File "/usr/code/autotest/client/shared/error.py", line 138, in new_fn
    return fn(*args, **kwargs)
  File "/usr/code/autotest/client/tests/virt/qemu/tests/boot_from_device.py", line 112, in run_boot_from_device
    vm.create()
  File "/usr/code/autotest/client/shared/error.py", line 138, in new_fn
    return fn(*args, **kwargs)
  File "/usr/code/autotest/client/tests/virt/virttest/qemu_vm.py", line 1769, in create
    raise virt_vm.VMImageMissingError(iso)
VMImageMissingError: CD image file not found: '/tmp/test.iso'

This patch could fix this.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.cfg.boot_from_device: Update bootindex.

Not first index is 0, so update bootindex to 0, if we want to boot from it
firstly.

Signed-off-by: Feng Yang <fyang@redhat.com>