pci_iov: Support dynamic subordinate bus growth during VF creation

Certain SR-IOV devices enumerate Virtual Functions (VFs) on a different
PCIe bus than their parent Physical Function (PF). In such cases, the
default subordinate bus range assigned by BIOS may be insufficient to cover
all VFs.

This patch dynamically expands the subordinate bus range by:

• Allocating additional bus numbers using bus_alloc_resource() when VFs are initialized
• Releasing the reserved bus range during VF deletion via bus_release_resource()

Reviewed by: jhb