tools: Install pv bootloaders in libexec rather than /usr/bin

tools: Install pv bootloaders in libexec rather than /usr/bin

pygrub and xenpvnetboot are meant to be run by tools, and not by the user,
and thus should be in /usr/lib/xen/bin rather than /usr/bin.

Because most config files will still have an absolute path pointing to
/usr/bin/pygrub, make a symbolic link that we will deprecate.

Signed-off-by: George Dunlap <george.dunlap@eu.ctirix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>