More error messages for passthru.

[jhb]

Summary

Always emit an error message on passthru configuration errors.

Previously, many errors (such as the PCI device not being attached
to the ppt(4) driver) resulted in bhyve silently exiting without
starting the virtual machine. Now any errors encountered when
configuring a virtual slot for a PCI passthru device should be noted
on stderr.

Test Plan

• Try to pass through a device that doesn't exist or isn't attached to ppt(4).

[neel]

Looks good.