pci: Accomodate 64 bit BARs in pci_dev::resource[]

Array pci_dev::resource[] is ambiguous wrt what is
actually stored in its elements and how to interpret
the index into the array.

It is simple in case a device has only 32-bit BARs -
an element pci_dev::resource[bar_num] contains the
decoded address of BAR # bar_num.

But what if a device has 64-bit BAR starting at bar_num?

Curretnly pci_dev::resource[bar_num] contains the decoded
address of the BAR, while pci_dev::resource[bar_num + 1]
contains 0. That makes meaning of (bar_num + 1) index
difficult to understand.

On the one hand a testcase should not address a 64-bit BAR
using high part BAR number. Particularly, when it tries to
obtan BAR size or address. On the other hand a testcase
should be able to access a low and high parts separately
if it needs to for whatever reason. The rest of the API
allows that as well.

pci_dev::resource[] contains decoded 32/64-bit BAR addresses,
not low/high parts of underlying 32-bit device BARs. Yet,
indexes into this array correspond to raw 32-bit BARs in the
PCI device configuration space. Thus, a question arises -
what should be stored in array elements that correspond to
high-parts of 64-bit BARs? Zero is particularly bad choice,
because:
  - it is a valid address in PIO address space, so it can not
    stand for "no value" or NULL or whatever marker could be
    used to indicate a high part;
  - the high part of underlying 64-bit address is (could be)
    non-zero. So there is inconsistency also;

By placing the same 64-bit address in both bar_num and
(bar_num + 1) elements the ambiguity is less striking,
since:
  - the meaning of bar_num kept consistent with the rest
    of the functions (where it refers 32-bit BAR in terms
    of the device configuration address space);
  - pci_dev::resource[bar_num + 1] contains a valid address
    rather than vague value of 0.
  - both bar_num and (bar_num + 1) indexes refer to the
    same 64-bit BAR and therefore return the same address;
    The notion of low and high parts of a 64-bit address
    is ignored, but that is fine, since pci_dev::resource[]
    contain only full addresses;

Cc: Thomas Huth <thuth@redhat.com>
Cc: Andrew Jones <drjones@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>