tests/qtest/vhost-user-blk-test: don't abort all qtests on missing envar

tests/qtest/vhost-user-blk-test: don't abort all qtests on missing envar

This test requires environment variable QTEST_QEMU_STORAGE_DAEMON_BINARY
to be defined for running. If not, it would immediately abort all qtests
and prevent other, unrelated tests from running.

To fix that, just skip vhost-user-blk-test instead and log a message
about missing environment variable.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <E1oybRD-0005D5-5r@lizzy.crudebyte.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>