qapi: update the qobject visitor to use QNUM_U64

Switch to use QNum/uint where appropriate to remove i64 limitation.

The input visitor will cast i64 input to u64 for compatibility
reasons (existing json QMP client already use negative i64 for large
u64, and expect an implicit cast in qemu).

Note: before the patch, uint64_t values above INT64_MAX are sent over
json QMP as negative values, e.g. UINT64_MAX is sent as -1. After the
patch, they are sent unmodified.  Clearly a bug fix, but we have to
consider compatibility issues anyway.  libvirt should cope fine,
because its parsing of unsigned integers accepts negative values
modulo 2^64.  There's hope that other clients will, too.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20170607163635.17635-12-marcandre.lureau@redhat.com>
[check_native_list() tweaked for consistency with signed case]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
