manual: Small formatting fixes
manual: Small formatting fixes

The XML namespace to use with libvirt when adding custom QEMU arguments
looks better in a monospace font.
asciidoc will also replace the '->' in '<->' with an arrow character,
which looks bad. This commit reformulates the sentence to avoid the use
of <->

fgouget/spice,fgouget/spice,SPICE/spice,SPICE/spice,fgouget/spice,fgouget/spice,SPICE/spice