Fix vcpu hotplug bug: transfer vcpu_avail hex string to qemu

Fix vcpu hotplug bug: transfer vcpu_avail hex string to qemu

Currently qemu has a bug: When maxvcpus > 64, qemu will get wrong
vcpu bitmap (s->cpus_sts[i]) since it only get bitmap from a long
variable.

This patch, cooperate with another qemu patch, is to fix this bug.
This patch transfer a vcpu_avail string in a hex string format, so
that at qemu side it's more easier to get vcpu bitmap from hex string,
especially when many vcpus, like more than 64.

(Also update QEMU_TAG for matching qemu-side update)

Signed-off-by: Liu, Jinsong <jinsong.liu@intel.com>