Fix exception when creating  new pool definition

- fix following rspec error

1) VagrantPlugins::ProviderKvm::Driver::Driver#import does not raise execption

Failure/Error: expect do
expected no Exception, got #<Libvirt::Error: Call to virStoragePoolBuild failed: Requested operation is not valid: storage pool 'vagrant' is already active> with backtrace:

 /home/travis/build/miurahr/vagrant-kvm/lib/vagrant-kvm/driver/driver.rb:263:in `build'
 /home/travis/build/miurahr/vagrant-kvm/lib/vagrant-kvm/driver/driver.rb:263:in `rescue in init_storage_directory'

Signed-off-by: Hiroshi Miura <miurahr@linux.com>