cpu: vmexit: Handle KVM_EXIT_UNKNOWN exit reason correctly

The return value for the KVM_RUN ioctl is confusing and has led to
errors in different kernel exit handlers. A return value of 0 indicates
a return to the VMM, whereas a return value of 1 indicates resuming
execution in the guest. Some handlers mistakenly return 0 to force a
return to the guest.

This worked in kvmtool because the exit_reason defaulted to
0 (KVM_EXIT_UNKNOWN), and kvmtool did not error out on an unknown exit
reason. However, forcing a KVM panic on an unknown exit reason would
help catch these bugs early.

Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>