conf: improve error log when PCI devices don't match requested controller

conf: improve error log when PCI devices don't match requested controller

The virDomainPCIAddressFlagsCompatible() error logs report that a
device required a controller that accepted standard PCI endpoint
devices, or PCI Express endpoint devices, and if hotplug was required
by the configuration but not provided by the selected controller. But
the wording of the error messages was apparently confusing (according
to the bugzilla report referenced below). On top of that, if the
device was something other than an endpoint device (e.g. a
pcie-switch-downstream-port) the error message was a complete punt -
it would just say that the flags were incorrect.

This patch makes the messages for PCI/PCIe endpoint and hotplug
requirements more clear, and also specifically indicates what was the
device type when it is other than an endpoint device.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1363627