We already have virAsprintf, so picking a similar name helps for
seeing a similar purpose.  Furthermore, the prefix V before printf
generally implies 'va_list', even though this variant was '...', and
the old name got in the way of adding a new va_list version.

global rename performed with:

$ git grep -l virBufferVSprintf \
  | xargs -L1 sed -i 's/virBufferVSprintf/virBufferAsprintf/g'

then revert the changes in ChangeLog-old.

Make virtTestLoadFile allocate the buffer to read the file into.

Fix logic error in virtTestLoadFile, stop reading on the first empty line.

Use virFileReadLimFD in virtTestCaptureProgramOutput to avoid manual
buffer handling.

A few of the tests were missing basic sanity checks, while most
of them were doing copy-and-paste initialization (in fact, some
of them pasted the argc > 1 check more than once!).  It's much
nicer to do things in one common place, and minimizes the size of
the next patch that fixes getcwd usage.

* tests/testutils.h (EXIT_AM_HARDFAIL): New define.
(progname, abs_srcdir): Define for all tests.
(VIRT_TEST_MAIN): Change callback signature.
* tests/testutils.c (virtTestMain): Do more common init.
* tests/commandtest.c (mymain): Simplify.
* tests/cputest.c (mymain): Likewise.
* tests/esxutilstest.c (mymain): Likewise.
* tests/eventtest.c (mymain): Likewise.
* tests/hashtest.c (mymain): Likewise.
* tests/networkxml2xmltest.c (mymain): Likewise.
* tests/nodedevxml2xmltest.c (myname): Likewise.
* tests/nodeinfotest.c (mymain): Likewise.
* tests/nwfilterxml2xmltest.c (mymain): Likewise.
* tests/qemuargv2xmltest.c (mymain): Likewise.
* tests/qemuhelptest.c (mymain): Likewise.
* tests/qemuxml2argvtest.c (mymain): Likewise.
* tests/qemuxml2xmltest.c (mymain): Likewise.
* tests/qparamtest.c (mymain): Likewise.
* tests/sexpr2xmltest.c (mymain): Likewise.
* tests/sockettest.c (mymain): Likewise.
* tests/statstest.c (mymain): Likewise.
* tests/storagepoolxml2xmltest.c (mymain): Likewise.
* tests/storagevolxml2xmltest.c (mymain): Likewise.
* tests/virbuftest.c (mymain): Likewise.
* tests/virshtest.c (mymain): Likewise.
* tests/vmx2xmltest.c (mymain): Likewise.
* tests/xencapstest.c (mymain): Likewise.
* tests/xmconfigtest.c (mymain): Likewise.
* tests/xml2sexprtest.c (mymain): Likewise.
* tests/xml2vmxtest.c (mymain): Likewise.