Dell PowerMax SnapVx unlink fails as the linked device
is not yet fully defined.
This patch fixed the issue by checking the new configuration
'snapvx_unlink_symforce' and the linked device 'defined' status
to determine the value of 'symforce' in the payload of
SnapVx unlink restcall.
Closes-Bug: #2045230
Change-Id: I614f6aef2d4da76c417b4a143ab80e4a5f716dcd