base_cfg: Fix variable naming typo

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.utils_params: Improve message of missing key

This error message covers most cases of the missing config key thus
it allow us to use params[] for required configs instead of checking
and raising custom exceptions.

Changes from v1:
 * Raise a custom error, ParamNotFound, that inherits
   from error.TestNAError, that will make the tests
   that lack a mandatory param to be skipped.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Add unittests for utils_params

They'll come up handy, and will help to verify whether
the Params class is behaving like we intended.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
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>