bhyve/virtio-scsi: Unlimit max_sectors

The old default of 2 for max_sectors limits performance significantly
with guest systems where the virtio-scsi driver actually honors this
value.

As CTL doesn't seem to have any limit of the maximum size of a single
transfer, set this to INT32_MAX by default.

Reviewed by: corvink
Differential Revision: https://reviews.freebsd.org/D53466