The security policy point is the one I would check second, right after confirming the binary exists, because the error surfaces so misleadingly.
On a distribution with mandatory access control enabled, sharing a directory from an unusual location — a home directory, an external mount, anything outside the paths the policy expects for virtual machine storage — gets denied. The manager reports a generic start failure and the useful message is only in the host's audit log.
Worth knowing the shape of the fix: either relabel the directory so the policy permits it, or share from a location the policy already allows. Turning the policy off entirely is the advice you will find most often and it is the worst of the options.