qdev: Use int32_t container for devfn property

Valid range for devfn is -1 to 255 (-1 for automatic assignment). We do
not currently validate this due to devfn being stored as a uint32_t.
This can lead to segfaults and other strange behavior.

We could technically just cast it to int32_t to implement the checking,
but this will not work for visitor-based setting where we may do additional
bounds-checking based on target container type, which is int32_t for this
case.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
