virsh: Fix docs for "virsh setmaxmem"
virsh: Fix docs for "virsh setmaxmem"

https://bugzilla.redhat.com/show_bug.cgi?id=924648

The docs assumed the command works always for QEMU and other
hypervisors. As this is done using the balloon mechainism live increase
of the maximum memory limit isn't supported. Fix the docs to mention
this limitation.
(cherry picked from commit b88831f75185bb5d9e1a8f389a3af0892990f072)

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