configure: unify handling of xyzdir variables

Making an xyzdir variable for each directory prepares for the next
patches introducing config-host.h defines and configure options for them.
It also fixes the problem where overriding prefix at "make install"
time did not override it for sysconfdir.

Removes some of the differences between sysconfdir and other variables,
the rest will go away later.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
