pci_iov: Reuse downstream bridge bus window if it already covers VF bus

If the parent bridge's [secondary, subordinate] window already covers
the VF bus (e.g., programmed by BIOS or a prior PF), skip allocating
PCI_RES_BUS. This avoids a duplicate rman allocation in the multi-PF
case while still allocating when growth is actually needed.

Reviewed by: ssaxena
Differential Revision: https://reviews.freebsd.org/D52163
MFC After: 1 week