selftests: kvm: move declaration at the beginning of main()

Placing a declaration of evt_reset is pedantically invalid
according to the C standard.  While GCC does not really care
and only warns with -Wpedantic, clang ignores the declaration
altogether with an error:

x86_64/xen_shinfo_test.c:965:2: error: expected expression
        struct kvm_xen_hvm_attr evt_reset = {
        ^
x86_64/xen_shinfo_test.c:969:38: error: use of undeclared identifier evt_reset
        vm_ioctl(vm, KVM_XEN_HVM_SET_ATTR, &evt_reset);
                                            ^

Reported-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Reported-by: Sean Christopherson <seanjc@google.com>
Fixes: a79b53aa ("KVM: x86: fix deadlock for KVM_XEN_EVTCHN_RESET", 2022-12-28)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>