monitor: Use accel_find("kvm") instead of kvm_available()

The kvm_available() function reports whether KVM support was
compiled into the QEMU binary; it returns the value of the
CONFIG_KVM define.

The only place in the codebase where we use this function is
in qmp_query_kvm(). Now that accelerators are based on QOM
classes we can instead use accel_find("kvm") and remove the
kvm_available() function.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210730105947.28215-3-peter.maydell@linaro.org