Add test for virsh blockcopy - v2

This patch for virsh blockcopy command testing. 

For v2:
Rewrite the tests according the feedback from pull request 279, but as i'm not push the patches in time, the old pull request was closed, so i open a new one.

Defined test set:
   1:  virsh_blockcopy.positive_test.no_option
   2:  virsh_blockcopy.positive_test.reuse_external
   3:  virsh_blockcopy.positive_test.raw_format
   4:  virsh_blockcopy.positive_test.min_bandwidth
   5:  virsh_blockcopy.positive_test.timeout_check
   6:  virsh_blockcopy.positive_test.shallow_option
   7:  virsh_blockcopy.positive_test.wait_option
   8:  virsh_blockcopy.positive_test.pivot_option
   9:  virsh_blockcopy.positive_test.finish_option
  10:  virsh_blockcopy.positive_test.pivot_async_option
  11:  virsh_blockcopy.positive_test.finish_async_option
  12:  virsh_blockcopy.negative_test.invalid_external_dest
  13:  virsh_blockcopy.negative_test.invalid_bandwidth
  14:  virsh_blockcopy.negative_test.miss_wait_option
  15:  virsh_blockcopy.negative_test.invalid_mix_option
  16:  virsh_blockcopy.negative_test.persistent_domain

Signed-off-by: Yanbing Du <ydu@redhat.com>