qapi: Add Visitor interfaces for uint*_t and int*_t

This adds visitor interfaces for fixed-width integers types.
Implementing these in visitors is optional, otherwise we fall back to
visit_type_int() (int64_t) with some additional bounds checking to avoid
integer overflows for cases where the value fetched exceeds the bounds
of our target C type.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[LE: exclude negative values in uint*_t Visitor interfaces]
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
[AF: Merged fix by Laszlo]
Signed-off-by: Andreas Färber <afaerber@suse.de>
