virtio-modern: fix PCI common read/write functions on big endian targets

Virtio modern has the common data organized in little endian, but
on powerpc64 BE it was reading and writing in the wrong endian.

Submitted by: Leonardo Bianconi <leonardo.bianconi@eldorado.org.br>
Reviewed by: bryanv, alfredo
Sponsored by: Eldorado Research Institute (eldorado.org.br)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28947