When using IOPORT with pci_host_generic we are missing setting the rman_end()
which leads to end being before start and thus a signed extended very large
number of size later on, which kva_alloc() will fail upon and we will panic.
Add the missing call.

Debugged with: andrew
Reviewed by: br, andrew
Sponsored by: DARPA/AFRL
Found: while using virtio with gem5
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D6337