tools: hv: fcopy: set 'error' in case an unknown operation was requested

'error' variable is left uninitialized in case we see an unknown operation.
As we don't immediately return and proceed to pwrite() we need to set it
to something, HV_E_FAIL sounds good enough.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>