target/i386: Implement mc->kvm_type() to get VM type

KVM is introducing a new API to create confidential guests, which
will be used by TDX and SEV-SNP but is also available for SEV and
SEV-ES.  The API uses the VM type argument to KVM_CREATE_VM to
identify which confidential computing technology to use.

Since there are no other expected uses of VM types, delegate
mc->kvm_type() for x86 boards to the confidential-guest-support
object pointed to by ms->cgs.

For example, if a sev-guest object is specified to confidential-guest-support,
like,

  qemu -machine ...,confidential-guest-support=sev0 \
       -object sev-guest,id=sev0,...

it will check if a VM type KVM_X86_SEV_VM or KVM_X86_SEV_ES_VM
is supported, and if so use them together with the KVM_SEV_INIT2
function of the KVM_MEMORY_ENCRYPT_OP ioctl. If not, it will fall back to
KVM_SEV_INIT and KVM_SEV_ES_INIT.

This is a preparatory work towards TDX and SEV-SNP support, but it
will also enable support for VMSA features such as DebugSwap, which
are only available via KVM_SEV_INIT2.

Co-developed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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