As suggested by Anthony, this patch encapsulates the sequence "save
cpu_single_env, temporarily drop qemu_mutex, restore cpu_single_env" for
condition variables in a helper function. It also adds a safety check to
the open-coded kvm_mutex_lock that the caller is not a vcpu thread (as
kvm_mutex_unlock clears cpu_single_env).

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Avi Kivity <avi@qumranet.com>