driver: Improve input validation to make code scanners happier

driver: Improve input validation to make code scanners happier

We trust the configuration files passed down to the driver already
because they define the isolation set up by the hypervisor and can
therefore screw up the system in various ways.

Nevertheless, we can and should improve basic consistency checks of
config fields that influences allocations and copy operations. This will
detect some corruptions/inconsistencies earlier and also satisfies the
Coverity scanner.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>