KVM: selftests: Move raw KVM_SET_USER_MEMORY_REGION helper to utils

Move set_memory_region_test's KVM_SET_USER_MEMORY_REGION helper to KVM's
utils so that it can be used by other tests.  Provide a raw version as
well as an assert-success version to reduce the amount of boilerplate
code need for basic usage.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220226001546.360188-26-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>