qemu-options: bring the kernel and image options together

qemu-options: bring the kernel and image options together

How to control the booting of QEMU is often a source of confusion for
users. Bring the options that control this together in the manual
pages and add some verbiage to describe when each option is
appropriate. This attempts to codify some of the knowledge expressed
in:

  https://stackoverflow.com/questions/58420670/qemu-bios-vs-kernel-vs-device-loader-file/58434837#58434837

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220725140520.515340-14-alex.bennee@linaro.org>