KVM: selftests: Use __KVM_SYSCALL_ERROR() to handle non-KVM syscall errors

Use __KVM_SYSCALL_ERROR() to report and pretty print non-KVM syscall and
ioctl errors, e.g. for mmap(), munmap(), uffd ioctls, etc...

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>