Following patch will update LIBVIRT_CHECK_(LIB|LIB_ALT|PKG) macros in
a way that you will be able to call a another macro as a fallback if
the first one fails.  To allow that, we need to move the
LIBVIRT_ARG_WITH out of those macro to not have two or more same lines
in output of "configure --help".
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

Make it much easier to test a configuration built without readline
support, by reusing our existing library probe machinery.  It gets
a bit tricky with readline, which does not provide a pkg-config
snippet, and which on some platforms requires one of several
terminal libraries as a prerequiste, but the end result should be
the same default behavior but now with the option to disable things.

* m4/virt-readline.m4 (LIBVIRT_CHECK_READLINE): Simplify by using
LIBVIRT_CHECK_LIB.
* tools/virsh.c: Convert USE_READLINE to WITH_READLINE.
Signed-off-by: NEric Blake <eblake@redhat.com>

A future patch will allow disabling readline; doing this in an
isolated file instead of configure.ac will make the task easier.

* configure.ac: Move readline code...
* m4/virt-readline.m4: ...here.
Signed-off-by: NEric Blake <eblake@redhat.com>