xenserver: Install into /usr instead of /root/vswitch.

xenserver: Install into /usr instead of /root/vswitch.

/usr is the standard location for installation, so use that instead of our
nonstandard location under /root.

This migrates everything except the kernel modules to /usr.  The kernel
modules will be migrated in an upcoming commit.

One possibly surprising change is that the manpages listed in the %files
section of vswitch-xen.spec not only moved but added .gz extensions.  This
seems to be because RPM automatically compresses manpages, but only if they
are installed in a standard system location.