docs: Fix installation of man8 pages

docs: Fix installation of man8 pages

c/s a430436 "docs: Support for generating man(8) pages" accidentally
failed to update to the install and clean rules for man8 pages, meaning
that c/s 7b21214 "docs: Move xentrace.8 to docs/man/xentrace.pod.8"
caused a packaging regression when it came to xentop.8

To avoid similar bugs in the future, move the generation of the build,
install and clean rules into the manpage metarule.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>