KVM: selftests: Add test coverage for guest_memfd without GUEST_MEMFD_FLAG_MMAP

KVM: selftests: Add test coverage for guest_memfd without GUEST_MEMFD_FLAG_MMAP

If a VM type supports KVM_CAP_GUEST_MEMFD_MMAP, the guest_memfd test will
run all test cases with GUEST_MEMFD_FLAG_MMAP set.  This leaves the code
path for creating a non-mmap()-able guest_memfd on a VM that supports
mappable guest memfds untested.

Refactor the test to run the main test suite with a given set of flags.
Then, for VM types that support the mappable capability, invoke the test
suite twice: once with no flags, and once with GUEST_MEMFD_FLAG_MMAP
set.

This ensures both creation paths are properly exercised on capable VMs.

Run test_guest_memfd_flags() only once per VM type since it depends only
on the set of valid/supported flags, i.e. iterating over an arbitrary set
of flags is both unnecessary and wrong.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
[sean: use double-underscores for the inner helper]
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20251003232606.4070510-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>