Instead of having a single function that probes the
architecture from the monitor and then sets a bunch of
basic capabilities based on it, have a separate function
for each part: virQEMUCapsInitQMPArch() only sets the
architecture, and virQEMUCapsInitQMPBasicArch() only sets
the capabilities.

This split will be useful later on, when we will want to
set basic capabilities from the test suite without having
to go through the pain of mocking the monitor.