KVM: x86: Define more SEV+ page fault error bits/flags for #NPF

Define more #NPF error code flags that are relevant to SEV+ (mostly SNP)
guests, as specified by the APM:

 * Bit 31 (RMP):   Set to 1 if the fault was caused due to an RMP check or a
                   VMPL check failure, 0 otherwise.
 * Bit 34 (ENC):   Set to 1 if the guest’s effective C-bit was 1, 0 otherwise.
 * Bit 35 (SIZEM): Set to 1 if the fault was caused by a size mismatch between
                   PVALIDATE or RMPADJUST and the RMP, 0 otherwise.
 * Bit 36 (VMPL):  Set to 1 if the fault was caused by a VMPL permission
                   check failure, 0 otherwise.

Note, the APM is *extremely* misleading, and strongly implies that the
above flags can _only_ be set for #NPF exits from SNP guests.  That is a
lie, as bit 34 (C-bit=1, i.e. was encrypted) can be set when running _any_
flavor of SEV guest on SNP capable hardware.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240228024147.41573-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>