qdev: accept both strings and integers for PCI addresses

Visitors allow a limited form of polymorphism.  Exploit it to support
setting the non-legacy PCI address property both as a DD.F string
and as an 8-bit integer.

The 8-bit integer form is just too clumsy, it is unlikely that we will
ever drop it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
