xend: fix options for assigned pci

xend: fix options for assigned pci

pci global options and per-device options for HVM device model have
been broken for some time, the patch tries to fix the problem. It:
    * maintains global options in xend, and merge it into
      per-device option when creating the backend
    * merge the global options also into the parameter of pci-ins
      dm-command

The second one is there because the backend is effectively skipped
in ioemu at present, ioemu solely relies on the parameter string to
create the device.

Cc: Simon Horman <horms@verge.net.au>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Qing He <qing.he@intel.com>