qemu: qemuDomainPMSuspendForDuration: Check availability of agent
qemu: qemuDomainPMSuspendForDuration: Check availability of agent

It requires a guest agent configured and running in the domain's guest
OS, So check qemu agent during qemuDomainPMSuspendForDuration().

Signed-off-by: Lin Ma <b672e1c95326414c5ec2f68d68657d26580eb0bc@suse.de>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Signed-off-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

jfehlig/libvirt,jfehlig/libvirt,libvirt/libvirt,zippy2/libvirt,zippy2/libvirt,nertpinx/libvirt,libvirt/libvirt,nertpinx/libvirt,olafhering/libvirt,jardasgit/libvirt,jardasgit/libvirt,jardasgit/libvirt,jfehlig/libvirt,nertpinx/libvirt,olafhering/libvirt,crobinso/libvirt,zippy2/libvirt,jardasgit/libvirt,jfehlig/libvirt,zippy2/libvirt,crobinso/libvirt,nertpinx/libvirt,olafhering/libvirt,nertpinx/libvirt,libvirt/libvirt,crobinso/libvirt,jardasgit/libvirt,olafhering/libvirt,crobinso/libvirt,libvirt/libvirt