x86: Add a kvm module parameter check for vmware_backdoors test

vmware_backdoors test fails if the kvm module parameter
enable_vmware_backdoor is not set to Y. Add a check before
running the test.

Suggested-by: Wei Huang <Wei.Huang2@amd.com>
Signed-off-by: Babu Moger <babu.moger@amd.com>
Reviewed-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>