Split get_device_and_disk() into two (#743181)

get_device_and_disk() basically calls libparted to get a PedDevice
object representing a disk device and a PedDisk object representing a
partition table.  Re-implement get_device_and_disk() using two separate
functions, get_device() and get_disk(), to get one of these objects
each.

No functionality changes with this commit.  It enables future commits to
incrementally add support for whole disk devices into GParted without
needing libparted to recognise the contents and create a virtual "loop"
partition table.

Bug 743181 - Add unpartitioned drive read-write support