qemu-doc: Do not hard-code the name of the QEMU binary

In our documentation, we use a mix of "$QEMU", "qemu-system-i386" and
"qemu-system-x86_64" when we give examples to the users how to run
QEMU. Some more consistency would be good here. Also some distributions
use different names for the QEMU binary (e.g. "qemu-kvm" in RHEL), so
providing more flexibility here would also be good. Thus let's define
some variables for the names of the QEMU command and use those in the
documentation instead: @value{qemu_system} for generic examples, and
@value{qemu_system_x86} for examples that only work with the x86
binaries.

Message-Id: <20190828093447.12441-1-thuth@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>