KVM: selftests: Fix size of memslots created by Xen tests

For better or worse, the memslot APIs take the number of pages, not the
size in bytes.  The Xen tests need 2 pages, not 8192 pages.

Fixes: 8d4e7e80

 ("KVM: x86: declare Xen HVM shared info capability and add test case")
Cc: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210210182609.435200-3-seanjc@google.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>