vl: Only choose enabled accelerators in configure_accelerators

vl: Only choose enabled accelerators in configure_accelerators

By choosing "tcg:kvm" when kvm is not enabled, we generate
an incorrect warning: "invalid accelerator kvm".

At the same time, use g_str_has_suffix rather than open-coding
the same operation.

Presumably the inverse is also true with --disable-tcg.

Fixes: 28a0961757fc
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>