As of 54289916 we learned libvirt to use UEFI for domains.
However, management applications may firstly query if libvirt
supports it. And this is where virConnectGetDomainCapabilities()
API comes handy.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

This new module holds and formats capabilities for emulator. If you
are about to create a new domain, you may want to know what is the
host or hypervisor capable of. To make sure we don't regress on the
XML, the formatting is not something left for each driver to
implement, rather there's general format function.

The domain capabilities is a lockable object (even though the locking
is not necessary yet) which uses reference counter.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>