KVM: Reject device ioctls from processes other than the VM's creator

commit ddba91801aeb5c160b660caed1800eb3aef403f8 upstream.

KVM's API requires thats ioctls must be issued from the same process
that created the VM.  In other words, userspace can play games with a
VM's file descriptors, e.g. fork(), SCM_RIGHTS, etc..., but only the
creator can do anything useful.  Explicitly reject device ioctls that
are issued by a process other than the VM's creator, and update KVM's
API documentation to extend its requirements to device ioctls.

Fixes: 852b6d57 ("kvm: add device control API")
Cc: <stable@vger.kernel.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>