qemu: monitor: Extract halted state to a bitmap indexed by cpu id

qemu: monitor: Extract halted state to a bitmap indexed by cpu id

We don't need to call qemuMonitorGetCPUInfo which is very inefficient to
get data required to update the vcpu 'halted' state.

Add a monitor helper that will retrieve the halted state and return it
in a bitmap so that it can be indexed easily.