KVM: selftests: fix max_guest_memory_test with more that 256 vCPUs

KVM: selftests: fix max_guest_memory_test with more that 256 vCPUs

max_guest_memory_test uses ucalls to sync with the host, but
it also resets the guest RIP back to its initial value in between
tests stages.

This makes the guest never reach the code which frees the ucall struct
and since a fixed pool of 512 ucall structs is used, the test starts
to fail when more that 256 vCPUs are used.

Fix that by replacing the manual register reset with a loop in
the guest code.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20240315143507.102629-1-mlevitsk@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>