util: storage: Parse 'lun' for iSCSI protocol from JSON as string or number

util: storage: Parse 'lun' for iSCSI protocol from JSON as string or number

While the QEMU QAPI schema describes 'lun' as a number, the code dealing
with JSON strings does not strictly adhere to this schema and thus
formats the number back as a string. Use the new helper to retrieve both
possibilities.

Note that the formatting code is okay and qemu will accept it as an int.

Tweak also one of the test strings to verify that both formats work
with libvirt.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1540290