nSVM: Reset the VMCB before every v1 test

Refresh the VMCB before every v1 test to fix bugs where tests neglect to
initialize the VMCB and end up taking a dependency on previous tests,
e.g. looking at you mode_test and next_rip.  This will also allow tests
to modify VMCB fields without having to do their own manual save/restore.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622210047.3691840-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>