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.

Reported by: wg
Reviewed by: araujo
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D18465