physmem: replace assertion with error

It is possible to start QEMU with a confidential-guest-support object
even in TCG mode.  While there is already a check in qemu_machine_creation_done:

    if (machine->cgs && !machine->cgs->ready) {
        error_setg(errp, "accelerator does not support confidential guest %s",
                   object_get_typename(OBJECT(machine->cgs)));
        exit(1);
    }

the creation of RAMBlocks happens earlier, in qemu_init_board(), if
the command line does not override the default memory backend with
-M memdev.  Then the RAMBlock will try to use guest_memfd (because
machine_require_guest_memfd correctly returns true; at least correctly
according to the current implementation) and trigger the assertion
failure for kvm_enabled().  This happend with a command line as
simple as the following:

    qemu-system-x86_64 -m 512 -nographic -object sev-snp-guest,reduced-phys-bits=48,id=sev0 \
       -M q35,kernel-irqchip=split,confidential-guest-support=sev0
    qemu-system-x86_64: ../system/physmem.c:1871: ram_block_add: Assertion `kvm_enabled()' failed.

Cc: Xiaoyao Li <xiaoyao.li@intel.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250217120812.396522-1-pbonzini@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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