The mutex and condition variable were never initialized, causing
-mem-prealloc to abort with an assertion failure.

Fixes: 037fb5eb
Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Cc: bauerchen <bauerchen@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>