KVM: selftests: Add ex_str() to print human friendly name of exception vectors

KVM: selftests: Add ex_str() to print human friendly name of exception vectors

Steal exception_mnemonic() from KVM-Unit-Tests as ex_str() (to keep line
lengths reasonable) and use it in assert messages that currently print the
raw vector number.

Co-developed-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-45-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>