If there's no PCI host bridge with ISA then check for PCI host bridge with
alias "pci0" (first PCI host bridge) and if it exists then choose it as the
primary PCI host bridge.

This makes choice of primary PCI host bridge more stable across boots and
updates as the last fallback candidate for primary PCI host bridge (if
there is no choice) is selected arbitrary.
Signed-off-by: NPali Rohár <pali@kernel.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220820123327.20551-1-pali@kernel.org