qemu: process: Update current balloon state to maximum on vm startup

qemu: process: Update current balloon state to maximum on vm startup

After libvirt issues the balloon resize command, the current balloon
size needs to be changed to the maximum memory size since the vCPUs were
not started and thus the balloon driver could not return the memory.

Since GetXMLDesc and other APIs return the balloon size without updating
it in case they are not able to obtain the job and the memory balloon
does not support the asynchronous event the sizing might be incorrect.