KVM: selftests: Fix hex vs. decimal snafu in Xen test

The Xen shinfo selftest uses '40' when setting the GPA of the vCPU info
struct, but checks for the result at '0x40'.  Arbitrarily use the hex
version to resolve the bug.

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: <20210210182609.435200-4-seanjc@google.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>