qemu: Clarify QEMU_CAPS_KVM

qemu: Clarify QEMU_CAPS_KVM

This capability is documented as having one meaning (whether
KVM is enabled by default) but is actually assigned two other
meanings over its life: whether the query-kvm QMP command is
available at first, and later on whether KVM is usable / was
used during probing.

Since the query-kvm QMP command was available in 1.5.0, we
can avoid probing for it; additionally, we can simplify the
logic by setting the flag when it applies instead of initially
setting it and then clearing it when it doesn't.

The flag's description is also updated to reflect reality.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>