xen_pt_msi.c: Check for xen_host_pci_get_* failures in xen_pt_msix_init()

Check the return status of the xen_host_pci_get_* functions we call in
xen_pt_msix_init(), and fail device init if the reads failed rather than
ploughing ahead. (Spotted by Coverity: CID 777338.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
