target/ppc: Clarify the meaning of return values in kvm_handle_debug

The kvm_handle_debug function can return 0 to go back into the guest
or return 1 to notify the gdbstub thread and pass control to GDB.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Message-Id: <20200110151344.278471-2-farosas@linux.ibm.com>
Tested-by: Leonardo Bras <leonardo@ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>