qemu_process: Fix theoretical overflow in uint to bool typecast

qemu_process: Fix theoretical overflow in uint to bool typecast

The qemuPrepareNVRAM() function accepts three arguments and the
last one being a boolean type. However, when the function is
called from qemuProcessPrepareHost() the argument passed is a
result of logical and of @flags (unsigned int) and
VIR_QEMU_PROCESS_START_RESET_NVRAM value. In theory this is
unsafe to do because if the value of the flag is ever changed
then this expression might overflow. Do what we do elsewhere:
double negation.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>