qemu_agent: Report error class at least

qemu_agent: Report error class at least

Currently, qemu GA is not providing 'desc' field for errors like
we are used to from qemu monitor. Therefore, we fall back to this
general 'unknown error' string. However, GA is reporting 'class' which
is not perfect, but much more helpful than generic error string.
Thus we should fall back to class firstly and if even no class
is presented, then we can fall back to that generic string.

Before this patch:
virsh # dompmsuspend --target mem f16
error: Domain f16 could not be suspended
error: internal error unable to execute QEMU command
'guest-suspend-ram': unknown QEMU command error

After this patch:
virsh # dompmsuspend --target mem f16
error: Domain f16 could not be suspended
error: internal error unable to execute QEMU command
'guest-suspend-ram': The command has not been found