guest agent: remove uneeded dependencies

guest agent: remove uneeded dependencies

This patch tries to cull any uneeded library dependencies from the guest
agent to improve portability across various distros. We do so by being
as explicit as possible about in-tree dependencies rather than relying
on existing *-obj-y targets, and by manually setting LIBS for the
qemu-ga target to avoid pulling in LIBS_TOOLS libraries discovered by
configure.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>