testutilsqemu: Introduce 'testQemuInsertRealCaps' helper

testutilsqemu: Introduce 'testQemuInsertRealCaps' helper

'testQemuInsertRealCaps' looks up and inserts real capabilities into the
capability 'file cache' for testing purposes. Effectively this helper
replaces following steps:

1) testQemuGetRealCaps
2) virFileCacheClear
3) qemuTestCapsCacheInsert

This helper doesn't copy the capabilities that are borrowed from it's
internal cache thus they must not be modified afterwards in contrast to
the above steps.

The use of this helper is in simple tests which require some form of
capabilities to parse a definition but don't care about doctoring them
in any way.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>