Skip reading existing partition table before creating a new one (#743181)

Creating a new partition table was getting libparted to read any
existing partition table before creating a new partition table on the
device.  This is an unnecessary step, and if the device didn't already
contain a partition table also printed this error from libparted:

    /dev/sdb: unrecognised disk label

Since get_device_and_disk() has been split into two, just call
get_device() instead to just populate the PedDevice object representing
the disk device.  Removes a small unnecessary step.

Bug 743181 - Add unpartitioned drive read-write support