block: driver should override flags in bdrv_open()

The BDRV_O_PROTOCOL flag should have an impact only if no driver is
specified explicitly. Therefore, if bdrv_open() is called with an
explicit block driver argument (either through the options QDict or
through the drv parameter) and that block driver is a protocol block
driver, BDRV_O_PROTOCOL should be set; if it is a format block driver,
BDRV_O_PROTOCOL should be unset.

While there was code to unset the flag in case a format block driver
has been selected, it only followed the bdrv_fill_options() function
call whereas the flag in fact needs to be adjusted before it is used
there.

With that change, BDRV_O_PROTOCOL will always be set if the BDS should
be a protocol driver; if the driver has been specified explicitly, the
new code will set it; and bdrv_fill_options() will only "probe" a
protocol driver if BDRV_O_PROTOCOL is set. The probing after
bdrv_fill_options() cannot select a protocol driver.

Thus, bdrv_open_image() to open BDS.file is never called if a protocol
BDS is about to be created. With that change in turn it is impossible to
call bdrv_open_common() with a protocol drv and file != NULL, which
allows us to remove the bdrv_swap() call.

This change breaks a test case in qemu-iotest 051:
"-drive file=t.qcow2,file.driver=qcow2" now works because the explicitly
specified "qcow2" overrides the BDRV_O_PROTOCOL which is automatically
set for the "file" BDS (and the filename is just passed down).
Therefore, this patch removes that test case.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
