usertools: add error on missing driver to bind

usertools: add error on missing driver to bind

A common user error is to forget driver to which the PCI devices should
be bound to. Currently, the error message in this case looks unhelpful
misleading and indecipherable to anyone but people who know how devbind
works.

Fix this by checking if the driver string is actually a valid device
string. If it is, we assume that the user has just forgot to specify the
driver, and display appropriate error. We also assume that no one will
name their driver in a format that looks like a PCI address, but that
seems like a reasonable assumption to make.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>