bhyve/vmrun.sh: pass all existing readable disks to bhyveload(8)

This is done if the installation mode is not requested or auto-detected.
This is useful when a kernel has to be loaded from multiple disks.
For example from a RAID-Z pool or a ZFS pool with multiple top level
vdevs.

Differential Revision: https://reviews.freebsd.org/D2723
Reviewed by: neel
MFC after: 8 days