Refactor PCI resource allocation

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.