all diskN aliases for providers (which more or less corresponds to how the
x86 version behaves) but instead probe only those listed in the boot-device
OFW environment variable. This has the following advantages:
- avoids otherwise unavoidable OFW warnings about failures to open disks
for which aliases exist but no actual hardware is connected
- avoids issues due to different diskN naming schemes
- aligns us with Solaris
MFC after: 3 days
614e309803