target/i386: kvm: do not access uninitialized variable on older kernels

target/i386: kvm: do not access uninitialized variable on older kernels

KVM support for AMX includes a new system attribute, KVM_X86_XCOMP_GUEST_SUPP.
Commit 19db68ca68 ("x86: Grant AMX permission for guest", 2022-03-15) however
did not fully consider the behavior on older kernels.  First, it warns
too aggressively.  Second, it invokes the KVM_GET_DEVICE_ATTR ioctl
unconditionally and then uses the "bitmask" variable, which remains
uninitialized if the ioctl fails.  Third, kvm_ioctl returns -errno rather
than -1 on errors.

While at it, explain why the ioctl is needed and KVM_GET_SUPPORTED_CPUID
is not enough.

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