KVM: selftests: Require KVM_CAP_USER_MEMORY2 for tests that create memslots

Explicitly require KVM_CAP_USER_MEMORY2 for selftests that create memslots,
i.e. skip selftests that need memslots instead of letting them fail on
KVM_SET_USER_MEMORY_REGION2.  While it's ok to take a dependency on new
kernel features, selftests should skip gracefully instead of failing hard
when run on older kernels.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/69ae0694-8ca3-402c-b864-99b500b24f5d@moroto.mountain
Suggested-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240430162133.337541-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>