x86: remove use of compiler's memset from emulator.c

Per Sean in discussion of the previous patch, "using the compiler's
memset() in kvm-unit-tests seems inherently dangerous since the tests
are often doing intentionally stupid things."

The string.h memset is already imported through libcflat.h, so let's use
that instead.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Jacob Xu <jacobhxu@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210511015016.815461-2-jacobhxu@google.com>