Don't race while creating a VCPU

Don't race while creating a VCPU

We hold qemu_mutex while machine->init() executes, which issues a VCPU create.
We need to make sure to not return from the VCPU creation until the VCPU
file descriptor is valid to ensure that APIC creation succeeds.

However, we also need to make sure that the VCPU thread doesn't start running
until the machine->init() is complete.  This is addressed today because the
VCPU thread tries to grab the qemu_mutex before doing anything interesting.
If we release qemu_mutex to wait for VCPU creation, then we open a window for
a race to occur.

This patch introduces two wait conditions.  The first lets the VCPU create
code that runs in the IO thread to wait for a VCPU to initialize.  The second
condition lets the VCPU thread wait for the machine to fully initialize before
running.

An added benefit of this patch is it makes the dependencies now explicit.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>