Use singleton usbdevice_list instead of usbdevice

This is a workaround since the default value of [] for usbdevice_list is
interpreted as a specified option in libxl and libxl disallows specifying both
the usbdevice and usbdevice_list options.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Don't exit when encountering exception from Domain.create_new

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Only export VNC over Unix sockets for upstream qemu

This special cases the change made by 4ca23dbe52a26e377131b626049fc4104c998530.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #21 from simonjbeaumont/cp-6238

Fix HVM domain creation using libxl