pci: add msi support for 32/64bit address

pci_cap_walk() is provided to allow walk through all the capability bits
for a specific PCI device. If a cap handler is provided, it'll be
triggered if the cap is detected along the cap walk.

MSI cap handler is the first one supported. We can add more cap handler
in the future.

Meanwhile, pci_setup_msi() API is provided to support basic 32/64 bit
address MSI setup.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>