KVM: x86/xen: Update self test for Xen PV timers

Add test cases for timers in the past, and reading the status of a timer
which has already fired.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20220309143835.253911-3-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>