xen/irq: If we fail during msi_capability_init return proper error code.

There are three different modes: PV, HVM, and initial domain 0. In all
the cases we would return -1 for failure instead of a proper error code.
Fix this by propagating the error code from the generic IRQ code.

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