LIBVIRT_ARG_WITH_ALT is more generic than LIBVIRT_ARG_WITH, which
is tailored at switching features on and off.

Rename the macros according to their intended purpose, and add
some documentation to help developers pick between the two.

Usage of AC_REQUIRE will mess with order how LIBVIRT_CHECK_* macros
are composed into configure.ac.  This ensures that the output of
configure --help is properly ordered and grouped into sections.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

The LIBVIRT_RESULT does a nice formatting of the output and ensures
that the format is unified.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

Move all the UML driver related routines into m4/virt-driver-uml.m4.