KVM: selftests: Add KVM_SET_USER_MEMORY_REGION2 helper

Add helpers to invoke KVM_SET_USER_MEMORY_REGION2 directly so that tests
can validate of features that are unique to "version 2" of "set user
memory region", e.g. do negative testing on gmem_fd and gmem_offset.

Provide a raw version as well as an assert-success version to reduce
the amount of boilerplate code need for basic usage.

Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20231027182217.3615211-33-seanjc@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>