json-parser: fix handling of large whole number values

Currently our JSON parser assumes that numbers lacking a fractional
value are integers and attempts to store them as QInt/int64 values. This
breaks in the case where the number overflows/underflows int64 values (which
is still valid JSON)

Fix this by detecting such cases and using a QFloat to store the value
instead.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Amos Kong <akong@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
