MFC r341705: Fix several iov handling bugs in bhyve virtio-scsi backend.
- buf_to_iov() does not use buflen parameter, allowing out of bound read.
- buf_to_iov() leaks memory if seek argument > 0.
- iov_to_buf() doesn't need to reallocate buffer for every segment.
- there is no point to use size_t for iov counts, int is more then enough.
- some iov function arguments can be constified.
- pci_vtscsi_request_handle() used truncate_iov() incorrectly, allowing
getting out of buffer and possibly corrupting data.
- pci_vtscsi_controlq_notify() written returned status at wrong offset.
- pci_vtscsi_controlq_notify() leaked one buffer per event.
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