vvfat: Fix qcow write target driver specification

vvfat: Fix qcow write target driver specification

First, bdrv_open_child() expects all options for the child to be
prefixed by the child's name (and a separating dot). Second,
bdrv_open_child() does not take ownership of the QDict passed to it but
only extracts all options for the child, so if a QDict is created for
the sole purpose of passing it to bdrv_open_child(), it needs to be
freed afterwards.

This patch makes vvfat adhere to both of these rules.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20160711135452.11304-1-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>