blockdev: Deprecate -drive with bogus interface type

Drives with interface types other than if=none are for onboard
devices.  Unfortunately, any such drives the board doesn't pick up can
still be used with -device, like this:

    $ qemu-system-x86_64 -nodefaults -display none -S -drive if=floppy,id=bogus,unit=7 -device ide-cd,drive=bogus -monitor stdio
    QEMU 5.0.50 monitor - type 'help' for more information
    (qemu) info block
    bogus: [not inserted]
	Attached to:      /machine/peripheral-anon/device[0]
	Removable device: not locked, tray closed
    (qemu) info qtree
    bus: main-system-bus
      type System
      [...]
	    bus: ide.1
	      type IDE
	      dev: ide-cd, id ""
--->		drive = "bogus"
		[...]
		unit = 0 (0x0)
      [...]

This kind of abuse has always worked.  Deprecate it:

    qemu-system-x86_64: -drive if=floppy,id=bogus,unit=7: warning: bogus if=floppy is deprecated, use if=none

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200622094227.1271650-9-armbru@redhat.com>