xml: allow scaled memory on input

xml: allow scaled memory on input

Output is still in kibibytes, but input can now be in different
scales for ease of typing.

* src/conf/domain_conf.c (virDomainParseMemory): New helper.
(virDomainDefParseXML): Use it when parsing.
* docs/schemas/domaincommon.rng: Expand XML; rename memoryKBElement
to memoryElement and update callers.
* docs/formatdomain.html.in (elementsMemoryAllocation): Document
scaling.
* tests/qemuxml2argvdata/qemuxml2argv-memtune.xml: Adjust test.
* tests/qemuxml2xmltest.c: Likewise.
* tests/qemuxml2xmloutdata/qemuxml2xmlout-memtune.xml: New file.