KVM: selftests: Add wrappers for mmap() and munmap() to assert success

KVM: selftests: Add wrappers for mmap() and munmap() to assert success

Add and use wrappers for mmap() and munmap() that assert success to reduce
a significant amount of boilerplate code, to ensure all tests assert on
failure, and to provide consistent error messages on failure.

No functional change intended.

Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20251003232606.4070510-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>