KVM: selftests: Create GUEST_MEMFD for relevant invalid flags testcases

KVM: selftests: Create GUEST_MEMFD for relevant invalid flags testcases

Actually create a GUEST_MEMFD instance and pass it to KVM when doing
negative tests for KVM_SET_USER_MEMORY_REGION2 + KVM_MEM_GUEST_MEMFD.
Without a valid GUEST_MEMFD file descriptor, KVM_SET_USER_MEMORY_REGION2
will always fail with -EINVAL, resulting in false passes for any and all
tests of illegal combinations of KVM_MEM_GUEST_MEMFD and other flags.

Fixes: 5d74316466f4 ("KVM: selftests: Add a memory region subtest to validate invalid flags")
Link: https://lore.kernel.org/r/20240222190612.2942589-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>