qemu: Implement support for VIR_DOMAIN_VCPU_AGENT in qemuDomainSetVcpusFlags
qemu: Implement support for VIR_DOMAIN_VCPU_AGENT in qemuDomainSetVcpusFlags

https://bugzilla.redhat.com/show_bug.cgi?id=924400

This patch adds support for agent-based cpu disabling and enabling to
qemuDomainSetVcpusFlags() API.
(cherry picked from commit d47eff88fe50e43a36671f6d8d0eeda52835d5e0)

Conflicts:
	src/qemu/qemu_driver.c: only unmatched context

mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2