The `rbdGetDeviceList()` function uses two very similar types for
converting krbd and NBD device information from JSON. There is no need
to use this distinction, and callers of `rbdGetDeviceList()` should not
need to care about it either.
By introducing a `deviceInfo` interface with Get-functions, the
`rbdGetDeviceList()` function becomes a little simpler, with a clearly
defined API for the returned list.
Signed-off-by: Niels de Vos <ndevos@ibm.com>