iotests: vmdk: Enable zeroed_grained=on by default

iotests: vmdk: Enable zeroed_grained=on by default

In order to avoid bitrot in the zero cluster code in VMDK, enable
zeroed_grain=on by default for the tests.

059 now unsets the default options because zeroed_grain=on works only
with some subformats and the test case tests many different subformats,
including those for which it doesn't work.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200430133007.170335-7-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>