KVM: selftests: Add helpers to read integer module params

Add helpers to read integer module params, which is painfully non-trivial
because the pain of dealing with strings in C is exacerbated by the kernel
inserting a newline.

Don't bother differentiating between int, uint, short, etc.  They all fit
in an int, and KVM (thankfully) doesn't have any integer params larger
than an int.

Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20240109230250.424295-24-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>