Refactor the certification validation code

Refactor the certification validation code

There is some commonality between the code for sanity checking
certs when initializing libvirt and the code for validating
certs during a live TLS session handshake. This patchset splits
up the sanity checking function into several smaller functions
each doing a specific type of check. The cert validation code
is then updated to also call into these functions

* src/rpc/virnettlscontext.c: Refactor cert validation code