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>

docs/specs/fw_cfg.rst

@@ -54,11 +54,11 @@ Data Register
 -------------
 
 * Read/Write (writes ignored as of QEMU v2.4, but see the DMA interface)
-* Location: platform dependent (IOport [#]_ or MMIO)
+* Location: platform dependent (IOport\ [#placement]_ or MMIO)
 * Width: 8-bit (if IOport), 8/16/32/64-bit (if MMIO)
 * Endianness: string-preserving
 
-.. [#]
+.. [#placement]
     On platforms where the data register is exposed as an IOport, its
     port number will always be one greater than the port number of the
     selector register. In other words, the two ports overlap, and can not