Rewrite some parts for clarity, elaborate the meaning of some of the XML
attributes.  And where necessary, distinguish that we're dealing with
two different XML documents here:

  - the domainCapabilities XML, to detect the host "hypervisor"
    (QEMU/KVM) capabilities, and what libvirt knows about them.

  - the guest XML definition, i.e. what features a guest can use, based
    on the capabilities (of QEMU and libvirt and the host) reported in
    the domainCapabilities XML.
Signed-off-by: NKashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>