This patch adds a visitor interface to Property.  This way, QOM will be
able to expose Properties that access a fixed field in a struct without
exposing also the everything-is-a-string "feature" of qdev properties.

Whenever the printed representation in both QOM and qdev (which is
typically the case for device backends), parse/print code can be reused
via get_generic/set_generic.  Dually, whenever multiple PropertyInfos
have the same representation in both the struct and the visitors the
code can be reused (for example among all of int32/uint32/hex32).

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>