kvm: x86: mmu: Use symbolic constants for EPT Violation Exit Qualifications

This change adds some symbolic constants for VM Exit Qualifications
related to EPT Violations and updates handle_ept_violation() to use
these constants instead of hard-coded numbers.

Signed-off-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>