docs: Get rid of the weird _005f links in the man page

docs: Get rid of the weird _005f links in the man page

The man page does not contain all the chapters from the System Emulation
Users Guide, so some of the links that we've put into the qemu options
descriptions can not be resolved and thus the link names are used in the
man pages instead. These link names currently contain weird "_005f" letters
in the middle and just do not make any sense for the users. To avoid this
situation, replace the link names with more descriptive, natural text.

Message-Id: <20201116145341.91606-1-thuth@redhat.com>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3
Buglink: https://bugs.launchpad.net/qemu/+bug/1453608
Signed-off-by: Thomas Huth <thuth@redhat.com>