qemu: Don't cache domCaps in virQEMUDriverGetDomainCapabilities()

qemu: Don't cache domCaps in virQEMUDriverGetDomainCapabilities()

Currently, whenever a domain capabilities is needed (fortunately,
after cleanup done by previous commits it is now only in
virConnectGetDomainCapabilities()), the object is stored in a
cache. But there is no invalidation mechanism for the cache
(except the implicit one - the cache is part of qemuCaps and thus
share its lifetime, but that is not enough). Therefore, if
something changes - for instance new firmware files are
installed, or old are removed these changes are not reflected in
the virConnectGetDomainCapabilities() output.

Originally, the caching was there because domCaps were used
during device XML validation and they were used a lot from our
test suite. But this is no longer the case. And therefore, we
don't need the cache and can construct fresh domCaps on each
virConnectGetDomainCapabilities() call.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1807198

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>