virnettlscontext: Resolve Coverity warnings (UNINIT)

virnettlscontext: Resolve Coverity warnings (UNINIT)

Coverity complained about the usage of the uninitialized cacerts in the
event(s) that "access(certFile, R_OK)" and/or "access(cacertFile, R_OK)"
fail the for loop used to fill in the certs will have indeterminate data
as well as the possibility that both failures would result in the
gnutls_x509_crt_deinit() call having a similar fate.

Initializing cacerts only would resolve the issue; however, it still
would leave the indeterminate action, so rather add a parameter to
the virNetTLSContextLoadCACertListFromFile() to pass the max size rather
then overloading the returned count parameter. If the the call is never
made, then we won't go through the for loops referencing the empty
cacerts