virtio: fix mips regression introduced by r357596

PowerPC support was fixed in r357596 by changing PCI bustag to BE as
part of the solution, but this caused regression on mips. This change
implements byte swapping of virtio PCI config area in the driver,
leaving lower layer untouched.

Submittnd by: Fernando Valle <fernando.valle@eldorado.org.br>
Reported by: arichardson
Reviewed by: alfredo, arichardson
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D25416