qemu: Implement virConnectGetDomainCapabilities

qemu: Implement virConnectGetDomainCapabilities

So far only information on disks and host devices are exposed in the
capabilities XML. Well, at least something. Even a new test is
introduced. The qemu capabilities are stolen from already existing
qemucapabilities test. There's one tricky point though. Functions that
checks host's KVM and VFIO capabilities, are impossible to mock
currently. So in the test, we are setting the capabilities by hand.

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