There is no need to initialize the IOMMU if no passthru devices have been
configured for bhyve to use.

Suggested by:	grehan@
- Reuse legacy_pcib_(read|write)_config() methods in the QPI pcib driver.
- Reuse legacy_pcib_alloc_msi{,x}() methods in the QPI and mptable pcib
  drivers.
Fix a typo.