qemu-config: Sections must consist of keys

In config_parse_qdict_section(), the QList returned by
qdict_array_split() is assumed to only contain QDicts. Currently, this
is true but it may (and will) change in the future. Therefore, check
whether the assumption actually holds.

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