qemu: Add domain support for VCPU halted state

qemu: Add domain support for VCPU halted state

Adding a field to the domain's private vcpu object to hold the halted
state information.
Adding two functions in support of the halted state:
- qemuDomainGetVcpuHalted: retrieve the halted state from a
  private vcpu object
- qemuDomainRefreshVcpuHalted: obtain the per-vcpu halted states
  via qemu monitor and store the results in the private vcpu objects

Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>
Reviewed-by: Bjoern Walk <bwalk@linux.vnet.ibm.com>
Reviewed-by: Hao QingFeng <haoqf@linux.vnet.ibm.com>
Signed-off-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>