KVM: selftests: Mark "guest_saw_irq" as volatile in xen_shinfo_test

Tag "guest_saw_irq" as "volatile" to ensure that the compiler will never
optimize away lookups.  Relying on the compiler thinking that the flag
is global and thus might change also works, but it's subtle, less robust,
and looks like a bug at first glance, e.g. risks being "fixed" and
breaking the test.

Make the flag "static" as well since convincing the compiler it's global
is no longer necessary.

Alternatively, the flag could be accessed with {READ,WRITE}_ONCE(), but
literally every access would need the wrappers, and eking out performance
isn't exactly top priority for selftests.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221013211234.1318131-17-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>