block: remove format defaults from QemuOpts in bdrv_create_file()

QemuOpts is usually created merging the QemuOptsList of format
and protocol. So, when the format calls bdr_create_file(), the 'opts'
parameter contains a QemuOptsList with a combination of format and
protocol default values.

The format properly removes its options before calling
bdr_create_file(), but the default values remain in 'opts->list'.
So if the protocol has options with the same name (e.g. rbd has
'cluster_size' as qcow2), it will see the default values of the format,
since for overlapping options, the format wins.

To avoid this issue, lets convert QemuOpts to QDict, in this way we take
only the set options, and then convert it back to QemuOpts, using the
'create_opts' of the protocol. So the new QemuOpts, will contain only the
protocol defaults.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20210308161232.248833-1-sgarzare@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>