vfio-pci: Test for extended config space

Having PCIe/PCI-X capability isn't enough to assume that there are
extended capabilities.  Both specs define that the first capability
header is all zero if there are no extended capabilities.  Testing
for this avoids an erroneous message about hiding capability 0x0 at
offset 0x100.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>