target/i386: SEV: use KVM_SEV_INIT2 if possible

target/i386: SEV: use KVM_SEV_INIT2 if possible

Implement support for the KVM_X86_SEV_VM and KVM_X86_SEV_ES_VM virtual
machine types, and the KVM_SEV_INIT2 function of KVM_MEMORY_ENCRYPT_OP.

These replace the KVM_SEV_INIT and KVM_SEV_ES_INIT functions, and have
several advantages:

- sharing the initialization sequence with SEV-SNP and TDX

- allowing arguments including the set of desired VMSA features

- protection against invalid use of KVM_GET/SET_* ioctls for guests
  with encrypted state

If the KVM_X86_SEV_VM and KVM_X86_SEV_ES_VM types are not supported,
fall back to KVM_SEV_INIT and KVM_SEV_ES_INIT (which use the
default x86 VM type).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>