KVM: selftests: Fix signedness issue with vCPU mmap size check

KVM: selftests: Fix signedness issue with vCPU mmap size check

Check that the return value of KVM_GET_VCPU_MMAP_SIZE is non-negative
before comparing with sizeof(kvm_run). If KVM_GET_VCPU_MMAP_SIZE fails,
it will return -1, and `-1 > sizeof(kvm_run)` is true, so the ASSERT
passes.

There are no other locations in tools/testing/selftests/kvm that make
the same mistake.

Signed-off-by: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20250711001742.1965347-1-jthoughton@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>