qemu-ga: Convert invocation documentation to rST

The qemu-ga documentation is currently in qemu-ga.texi in
Texinfo format, which we present to the user as:
 * a qemu-ga manpage
 * a section of the main qemu-doc HTML documentation

Convert the documentation to rST format, and present it to
the user as:
 * a qemu-ga manpage
 * part of the interop/ Sphinx manual

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Tested-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Message-id: 20190905131040.8350-1-peter.maydell@linaro.org