domain_conf: Use virDomainParseMemory more widely

domain_conf: Use virDomainParseMemory more widely

As reviewing patches upstream it occurred to me, that we have two
functions doing nearly the same: virDomainParseMemory which
expects XML in the following format:

  <memory unit='MiB'>1337</memory>

The other function being virDomainHugepagesParseXML expecting the
following format:

  <someElement size='1337' unit='MiB'/>

It wouldn't matter to have two functions handle two different
scenarios like this if we could only not copy code that handles
32bit arches around. So this code merges the common parts into
one by inventing new @units_xpath argument to
virDomainParseMemory which allows overriding the default location
of @unit attribute in XML. With this change both scenarios above
can be parsed with virDomainParseMemory.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>