KVM: x86/xen: Initialize Xen timer only once

KVM: x86/xen: Initialize Xen timer only once

Add a check for existing xen timers before initializing a new one.

Currently kvm_xen_init_timer() is called on every
KVM_XEN_VCPU_ATTR_TYPE_TIMER, which is causing the following ODEBUG
crash when vcpu->arch.xen.timer is already set.

ODEBUG: init active (active state 0)
object type: hrtimer hint: xen_timer_callbac0
RIP: 0010:debug_print_object+0x16e/0x250 lib/debugobjects.c:502
Call Trace:
__debug_object_init
debug_hrtimer_init
debug_init
hrtimer_init
kvm_xen_init_timer
kvm_xen_vcpu_set_attr
kvm_arch_vcpu_ioctl
kvm_vcpu_ioctl
vfs_ioctl

Fixes: 2f593423623f ("KVM: x86/xen: handle PV timers oneshot mode")
Cc: stable@vger.kernel.org
Link: https://syzkaller.appspot.com/bug?id=8234a9dfd3aafbf092cc5a7cd9842e3ebc45fc42
Reported-by: syzbot+e54f930ed78eb0f85281@syzkaller.appspotmail.com
Signed-off-by: Coleman Dietsch <dietschc@csp.edu>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220808190607.323899-2-dietschc@csp.edu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>