Fully initialize BARs with an address of 0 in pci_emul_alloc_bar()
instead of deferring some of that initialization to
pci_emul_assign_bar(). Now, the latter is only used to allocate an
initial address range for PCI BARs.

Note that this means that the pci_passthru model now overrides the
initial lobits after they are set removing the need for a workaround
in pci_emul_assign_bar().