ide: Change ide_init_drive() to require valid dinfo argument

IDEState members drive_serial_str and version are now left empty until
an actual drive is connected.  Before, they got a default value that
was overwritten when a drive got connected.  Doesn't matter, because
they're used only while a drive is connected.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
