qemu.qemu_vm: spice related bug fix

This patch fix several spice related script issue.

1.
19:20:54 ERROR| Original Traceback (most recent call last):
  File "/usr/code/autotest/client/tests/virt/virttest/qemu_vm.py", line 2240, in create
    self.devices = self.make_create_command()
  File "/usr/code/autotest/client/tests/virt/virttest/qemu_vm.py", line 1916, in make_create_command
    cmd += add_spice()
  File "/usr/code/autotest/client/tests/virt/virttest/qemu_vm.py", line 1034, in add_spice
    if (not os.path.exists(prefix) and
  File "/usr/lib64/python2.6/genericpath.py", line 18, in exists
    st = os.stat(path)
TypeError: coercing to Unicode: need string or buffer, NoneType found

2.
Traceback (most recent call last):
  File "/root/ypu/autotest_upstream_master/client/shared/test.py", line 426, in _exec
    _call_test_function(self.execute, *p_args, **p_dargs)
  File "/root/ypu/autotest_upstream_master/client/shared/test.py", line 853, in _call_test_function
    raise error.UnhandledTestFail(e)
UnhandledTestFail: Unhandled KeyError: 'spice_port'    [context: migrating 'virt-tests-vm1']
Traceback (most recent call last):
  File "/root/ypu/autotest_upstream_master/client/shared/test.py", line 846, in _call_test_function
    return func(*args, **dargs)
  File "/root/ypu/autotest_upstream_master/client/shared/test.py", line 299, in execute
    postprocess_profiled_run, args, dargs)
  File "/root/ypu/autotest_upstream_master/client/shared/test.py", line 216, in _call_run_once
    *args, **dargs)
  File "/root/ypu/autotest_upstream_master/client/shared/test.py", line 322, in run_once_profiling
    self.run_once(*args, **dargs)
  File "/root/ypu/autotest_upstream_master/client/tests/virt/virt.py", line 133, in run_once
    run_func(self, params, env)
  File "/root/ypu/autotest_upstream_master/client/tests/virt/qemu/tests/timedrift_with_migration.py", line 59, in run_timedrift_with_migration
    vm.migrate()
  File "/root/ypu/autotest_upstream_master/client/shared/error.py", line 138, in new_fn
    return fn(*args, **kwargs)
  File "/root/ypu/autotest_upstream_master/client/tests/virt/virttest/qemu_vm.py", line 3149, in migrate
    dest_port = clone.spice_options['spice_port']
KeyError: 'spice_port'

3.
simple the way to send spice related human/qmp monitor in vm.migrate().

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