i386/tdx: Get tdx_capabilities via KVM_TDX_CAPABILITIES

i386/tdx: Get tdx_capabilities via KVM_TDX_CAPABILITIES

KVM provides TDX capabilities via sub command KVM_TDX_CAPABILITIES of
IOCTL(KVM_MEMORY_ENCRYPT_OP). Get the capabilities when initializing
TDX context. It will be used to validate user's setting later.

Since there is no interface reporting how many cpuid configs contains in
KVM_TDX_CAPABILITIES, QEMU chooses to try starting with a known number
and abort when it exceeds KVM_MAX_CPUID_ENTRIES.

Besides, introduce the interfaces to invoke TDX "ioctls" at VCPU scope
in preparation.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250508150002.689633-6-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>