conf: fix omission of <driver> in domain dumpxml

conf: fix omission of <driver> in domain dumpxml

I noticed that depending on the <driver> attributes the user passed
in, the output may omit the <driver> element altogether.  For example,
the rerror_policy has had this problem since commit 4bb4109 in Oct
2011.  But in adding testsuite coverage to expose it, I found another
problem: the C code is just fine without a driver name, but the
XML validator required either a name or a cache mode.

* src/conf/domain_conf.c (virDomainDiskDefFormat): Update
conditional.
* docs/schemas/domaincommon.rng (diskDriver): Simplify.
* tests/qemuxml2argvdata/qemuxml2argv-disk-drive-copy-on-read.xml:
* tests/qemuxml2argvdata/qemuxml2argv-disk-drive-copy-on-read.args:
New files.
* tests/qemuxml2argvdata/qemuxml2argv-disk-drive-discard.xml:
Enhance test.
* tests/qemuxml2xmloutdata/qemuxml2xmlout-disk-drive-discard.xml:
Likewise.
* tests/qemuxml2argvtest.c (mymain): New test.
* tests/qemuxml2xmltest.c (mymain): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>