stream: tweak usage of bdrv_co_is_allocated

is_allocated_base has complex semantics that are not really usable
outside streaming.  Split the check in two parts, where the allocated
state for the top bs is moved to the caller.  The resulting function
is more generally useful.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
