KVM: selftests: Provide an API for getting a random bool from an RNG

Move memstress' random bool logic into common code to avoid reinventing
the wheel for basic yes/no decisions.  Provide an outer wrapper to handle
the basic/common case of just wanting a 50/50 chance of something
happening.

Link: https://lore.kernel.org/r/20240314185459.2439072-3-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>