xen/pciback: fix XEN_PCI_OP_enable_msix result

Prior to 2.6.19 and as of 2.6.31, pci_enable_msix() can return a
positive value to indicate the number of vectors (less than the amount
requested) that can be set up for a given device. Returning this as an
operation value (secondary result) is fine, but (primary) operation
results are expected to be negative (error) or zero (success) according
to the protocol. With the frontend fixed to match the XenoLinux
behavior, the backend can now validly return zero (success) here,
passing the upper limit on the number of vectors in op->value.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>