runqemu: enable kvm when use tap under sudo

runqemu: enable kvm when use tap under sudo

Enable kvm support automatically when use tap interface under sudo, the
thought is, if sudo works well, and qemu-native has been built with kvm
support, and kvm can be enabled, then enable it, it can be disabled by
nokvm option.

The previous command:
$ runqemu qemux86 kvm
may not work since it checks the current user's read/write permission on
/dev/kvm and /dev/vhost-net but failed without sudo, it can't use sudo
to check them since sudo is not a must, sudo is only required when need
create tap interface, this patch can enable it automatically.

Enable kvm can improve qemu's performance a lot, when test perl-5.22's
lib/warnings.t on qemux86-64, 70 times improved.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>