qemuCaps is tied to a binary on disk. domCaps is tied to a combo
of binary+machine+arch+virttype values. For the qemu driver this almost
entirely translates to a permutation of qemuCaps though

Upcoming patches want to use the domCaps data store at XML validate
time, but we need to cache the data so we aren't repeatedly
regenerating it.

Add a domCapsCache hash table to qemuCaps. This ensures that the domCaps
cache is blown away whenever qemuCaps needs to be regenerated. Similarly
when qemuCaps is invalidated, the next call to virQEMUCapsCacheLookup
will unref qemuCaps and free our cache as well.

Adjust virQEMUDriverGetDomainCapabilities to search the cache and add
to it if we don't find a hit.
Signed-off-by: NCole Robinson <crobinso@redhat.com>
Reviewed-by: NReviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>