KVM: PPC: Book3S HV: Allow DTL to be set to address 0, length 0

Commit 55b665b0

 ("KVM: PPC: Book3S HV: Provide a way for userspace
to get/set per-vCPU areas") includes a check on the length of the
dispatch trace log (DTL) to make sure the buffer is at least one entry
long.  This is appropriate when registering a buffer, but the
interface also allows for any existing buffer to be unregistered by
specifying a zero address.  In this case the length check is not
appropriate.  This makes the check conditional on the address being
non-zero.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>