iotests: Check for enabled drivers before testing them

It is possible to enable only a subset of the block drivers with the
"--block-drv-rw-whitelist" option of the "configure" script. All other
drivers are marked as unusable (or only included as read-only with the
"--block-drv-ro-whitelist" option). If an iotest is now using such a
disabled block driver, it is failing - which is bad, since at least the
tests in the "auto" group should be able to deal with this situation.
Thus let's introduce a "_require_drivers" function that can be used by
the shell tests to check for the availability of certain drivers first,
and marks the test as "not run" if one of the drivers is missing.

This patch mainly targets the test in the "auto" group which should
never fail in such a case, but also improves some of the other tests
along the way. Note that we also assume that the "qcow2" and "file"
drivers are always available - otherwise it does not make sense to
run "make check-block" at all (which only tests with qcow2 by default).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 20190823133552.11680-1-thuth@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

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