block: Fail gracefully with missing filename

This fixes a regression introduced in commit 2a05cbe42 ('block: Allow
block devices without files'):

$ qemu-system-x86_64 -drive driver=file
qemu-system-x86_64: block.c:892: bdrv_open_common: Assertion
`!drv->bdrv_needs_filename || filename != ((void *)0)' failed.

Now the respective check must be performed not only in bdrv_file_open(),
but also in bdrv_open().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
