hw/pci-bridge/pcie_pci_bridge: properly handle MSI unavailability case

QEMU with the pcie-pci-bridge device crashes if the guest board doesn't support MSI,
e.g. 'qemu-system-ppc64 -M prep -device pcie-pci-bridge'.
This is caused by wrong pcie-pci-bridge instantiation error handling. This patch fixes this issue
by falling back to legacy INTx if MSI is not available.
Also set the bridge's 'msi' property default value to 'auto' in order to trigger errors
only when user explicitly set msi=on.

Reported-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Aleksandr Bezzubikov <zuban32s@gmail.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
