Read physical subsystem device from vgpu config files

If the value of plugin0.pdev_id in the config file is of the form

"0x1234"

then store None as the physical subsystem device ID.

If the value is of the form

"0x1234:0x5678"

then store (Some 0x5678) as the physical subsystem device ID.

In both cases 0x1234 will be stored as the physical device ID.

Signed-off-by: John Else <john.else@citrix.com>
Refactor print_vgpu_conf into two functions

Signed-off-by: John Else <john.else@citrix.com>
Print the new psubdev_id on test failure

Signed-off-by: John Else <john.else@citrix.com>
Add tests of the vgpu config parser

Includes a couple of example config files.

Signed-off-by: John Else <john.else@citrix.com>