build: isolate core libvirt libs deps from xen runtime

build: isolate core libvirt libs deps from xen runtime

Generating libvirt packages per make rpm, "with-libxl=1" and "with-xen=1",
adds strict runtime dependencies per libxenlight for xen-libs package from
core libvirt-libs package.  This is not necessary and unfortunate since
those dependencies set demand to "xen-libs" package even when there's no
need for libvirt xen or libxl driver components.

This patch is to have two separate xenconfig lib tool libraries: one for
core libvirt (without XL), and a another that contains xl for libxl driver
(libvirt_driver_libxl_impl.la) which when loading the driver, loads the
remaining symbols (xen{Format,Parse}XL. For the user/sysadmin, this means
the xen dependencies are moved into libxl driver, instead of core libvirt.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Wim ten Have <wim.ten.have@oracle.com>
Reviewed-by:   Jim Fehlig <jfehlig@suse.com>