util: new function virNetDevMacVLanIsMacvtap()

util: new function virNetDevMacVLanIsMacvtap()

This function returns T if the given name is a macvtap device. This is
determined by 1) getting the ifindex of the device with that name (if
there is one), and 2) checking for existence of /dev/tapXX, where "XX"
is the ifindex learned in (1).

It's also possible to learn this by getting a netlink dump of the
interface and parsing through it to look for some attributes, but that
is complicated to figure out, takes longer to execute, and I'm lazy.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>