KVM: selftests: Add 'malloc' failure check in vcpu_save_state

There is a 'malloc' call in vcpu_save_state function, which can
be unsuccessful. This patch will add the malloc failure checking
to avoid possible null dereference and give more information
about test fail reasons.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20230322144528.704077-1-ivan.orlov0322@gmail.com


Signed-off-by: Sean Christopherson <seanjc@google.com>