KVM: selftests: Use alloc page helper for xAPIC IPI test

Use the common page allocation helper for the xAPIC IPI test, effectively
raising the minimum virtual address from 0x1000 to 0x2000.  Presumably
the test won't explode if it can't get a page at address 0x1000...

Cc: Peter Shier <pshier@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>