libqtest: Add qtest_[v]startf()

libqtest: Add qtest_[v]startf()

We have several callers that were formatting the argument strings
themselves; consolidate this effort by adding new convenience
functions directly in libqtest, and update some call-sites that
can benefit from it.

Note that the new functions qtest_startf() and qtest_vstartf()
behave more like qtest_init() (the caller must assign global_qtest
after the fact, rather than getting it implicitly set).  This helps
us prepare for future patches that get rid of the global variable,
by explicitly highlighting which tests still depend on it now.

Signed-off-by: Eric Blake <eblake@redhat.com>
[thuth: Dropped the hunks that do not apply cleanly to qemu master
 yet and added the missing g_free(args) in qtest_vstartf()]
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1508336428-20511-2-git-send-email-thuth@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>