bhyve: Simplify passthru_msix_addr()

It can use the passthru_mmio_map() helper function. Make that change,
and also make passthru_mmio_map() use EPRINTLN to fix formatting when
the guest console is stdio.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D55067