MAINTAINERS: Improve section headlines

MAINTAINERS: Improve section headlines

When scripts/get_maintainer.pl reports something like

    John Doe <jdoe@example.org> (maintainer:Overall)

the user is left to wonder *which* of our three "Overall" sections
applies: the one under "Guest CPU cores (TCG)", or the one under
"Guest CPU Cores (KVM)", or the one under "Usermode emulation".

Rename sections under

* "Guest CPU cores (TCG)" from "FOO" to "FOO TCG CPUs"

* "Guest CPU Cores (KVM)" from "FOO" to "FOO KVM CPUs"

* "Guest CPU Cores (Xen)" from "FOO" to "FOO Xen CPUs"

* "Architecture support" from "FOO" to "FOO general architecture
  support"

* "Usermode Emulation" from "Overall" to "Overall usermode emulation"

* "Tiny Code Generator (TCG)" from "FOO target" to "FOO TCG target",
  and from "Common code" to "Common TCG code"

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190606172408.18399-3-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>