docs: schema: make disk driver name attribute optional
docs: schema: make disk driver name attribute optional

/domain/devices/disk/driver/@name is not a required or mandatory
attribute according to formatdomain, and indeed it was agreed on
IRC that the attribute is "optional for input, recommended (but
not required) for output". Currently the schema requires the
attribute, causing virt-xml-validate to fail on disk config where
the driver name is not explicitly specified. E.g.

# cat test.xml | grep -A 5 cdrom
    <disk type='file' device='cdrom'>
      <driver type='raw'/>
      <target dev='hdb' bus='ide'/>
      <readonly/>
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
    </disk>

# virt-xml-validate test.xml
Relax-NG validity error : Extra element devices in interleave
test.xml:21: element devices: Relax-NG validity error : Element domain failed to validate content
test.xml fails to validate

Relaxing the name attribute to be optional fixes the validation

# virt-xml-validate test.xml
test.xml validates

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