xml: drop unenforced minimum memory limit from RNG

xml: drop unenforced minimum memory limit from RNG

The test domain allows <memory>0</memory>, but the RNG was stating
that memory had to be at least 4096000 bytes.  Hypervisors should
enforce their own limits, rather than complicating the RNG.

Meanwhile, some copy and paste had introduced some fishy constructs
in various unit tests.

* docs/schemas/domaincommon.rng (memoryKB, memoryKBElement): Drop
limit that isn't enforced in code.
* src/conf/domain_conf.c (virDomainDefParseXML): Require current
<= maximum.
* tests/qemuxml2argvdata/*.xml: Fix offenders.