qemu-options: Improve -global documentation

qemu-options: Improve -global documentation

Recent commit 3751d7c "vl: allow full-blown QemuOpts syntax for
-global" overloaded its existing argument syntax DRIVER.PROP=VALUE
with QemuOpts syntax.  Unambigious as long as no DRIVER contains '='.

Its documentation claims that "the two syntaxes are equivalent."
Improve it to spell out how exactly the old syntax gets desugared into
the new one.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>