KVM: selftests: Open code vcpu_run() equivalent in guest_printf test

KVM: selftests: Open code vcpu_run() equivalent in guest_printf test

Open code a version of vcpu_run() in the guest_printf test in anticipation
of adding UCALL_ABORT handling to _vcpu_run().  The guest_printf test
intentionally generates asserts to verify the output, and thus needs to
bypass common assert handling.

Open code a helper in the guest_printf test, as it's not expected that any
other test would want to skip _only_ the UCALL_ABORT handling.

Link: https://lore.kernel.org/r/20240719235107.3023592-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>