Interestingly I get the same "boot failed" error with Parallels 10.3. My VM (10.10.5 Server) ran fine until the update from Parallels 10.2.2 to 10.3. Reinstalling Parallels 10.2.2 didn't help, as the VM became unusable. Recovery Mode > Disk Utiliy did not find any errors. I was able to get a older VM backup running with Parallels 10.2.2. Now I tried to upgrade a copy of my VM to OS X 10.11 with Parallels 10.2.2. It looked promising, but about 4/5-ths into the installation, the progress bar stopped. It reminds me of the problems with Parallels and OS X 10.10 one year ago. I seems as if Windows is more thoroughly tested than OS X at Parallels.