blockdev: Fix drive_add for drives without media

blockdev: Fix drive_add for drives without media

Watch this:

    (qemu) drive_add 0 if=none
    (qemu) info block
    none0: type=hd removable=0 [not inserted]
    (qemu) drive_del none0
    Segmentation fault (core dumped)

add_init_drive() is confused about drive_init()'s failure modes, and
cleans up when it shouldn't.  This leaves the DriveInfo with member
opts dangling.  drive_del attempts to free it, and dies.

drive_init() behaves as follows:

* If it created a drive with media, it returns its DriveInfo.

* If it created a drive without media, it clears *fatal_error and
  returns NULL.

* If it couldn't create a drive, it sets *fatal_error and returns
  NULL.

Of its three callers:

* drive_init_func() is correct.

* usb_msd_init() assumes drive_init() failed when it returns NULL.
  This is correct only because it always passes option "file", and
  "drive without media" can't happen then.

* add_init_drive() assumes drive_init() failed when it returns NULL.
  This is incorrect.

Clean up drive_init() to return NULL on failure and only on failure.
Drop its parameter fatal_error.

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