qemu.tests: Use params[] for required params

This patch benefits from the better error message when key is missing
in config params. Instead of custom handling of missing param it
uses params[] directly.

I modified those params, where I was 100% certain that they are
required and doesn't have default value:
 * val = params.get(..) followed by if not val: raise...
 * int(params.get(...))
 * params.get(...).split()
 * commands which require string (eg. session.cmd(params.get(...)))

I didn't test all the tests before and after in order to verify
possible typos but I used pylint before and after, watching for
new errors (code rate before and after).

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
tests.cpu_hotplug: Fix a bracket typo

int(params.get(...), $int) should be probably default value of ...
=> int(params.get(..., $int))

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>