Refactor PCI resource allocation

Refactor PCI resource allocation code to allow a request for a
memory-mapped I/O window that is a multiple of a requested size.
This is needed by the SR-IOV code because the VF BARs are all
allocated contiguously. We can't just allocate a resource that is
a multiple of a single VF BAR because the size of an allocation
implies its alignment requirement.

Differential Revision: https://reviews.freebsd.org/D71
Reviewed by: jhb
MFC after: 1 month
Sponsored by: Sandvine Inc.