conf, docs: Add support for coalesce setting(s)

conf, docs: Add support for coalesce setting(s)

We are currently parsing only rx/frames/max because that's the only
value that makes sense for us.  The tun device just added support for
this one and the others are only supported by hardware devices which
we don't need to worry about as the only way we'd pass those to the
domain is using <hostdev/> or <interface type='hostdev'/>.  And in
those cases the guest can modify the settings itself.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>