build: fix regression in requiring yajl for new enough qemu

build: fix regression in requiring yajl for new enough qemu

Jonathan Lebon reported an issue to me off-list about his build
failing to use qemu because he failed to install yajl-devel.  But
I recalled specifically tweaking configure.ac to die in that
situation (commits 350583c, ba9c38b).  After a bit more
head-scratching, we found the cause of the regression: commit
654c709 rearranged things so that the qemu version check now
occurs before AC_ARG_WITH has had a chance to set either
$with_qemu or $with_yajl.

Coincidentally, this fix aligns with a documentation patch that
was just posted to the autoconf mailing list :)
http://thread.gmane.org/gmane.comp.sysutils.autoconf.patches/8324

* m4/virt-lib.m4 (LIBVIRT_CHECK_LIB, LIBVIRT_CHECK_LIB_ALT)
(LIBVIRT_CHECK_PKG): Populate defaults earlier.
* configure.ac (AC_ARG_WITH): Likewise for drivers.

Signed-off-by: Eric Blake <eblake@redhat.com>