docs: use consistent markup for footnotes

docs: use consistent markup for footnotes

Unfortunately, the definition of the footnote syntax requires
the author to use the awkward escaped space "\ " in the really common
case of "footnote marker at end of word or sentence"; and in fact the rST
documentation's examples of footnote syntax contain only artificial
examples that do *not* use the syntax.  This resulted in ugly rendering
of footnotes throughout QEMU's documentation.  Ensure the space is escaped
whenever the footnote must attach to the preceding word, and also use
a named reference for clarity.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>