vfio/pci: Make capability related functions return bool

vfio/pci: Make capability related functions return bool

The functions operating on capability don't have a consistent return style.

Below functions are in bool-valued functions style:
vfio_msi_setup()
vfio_msix_setup()
vfio_add_std_cap()
vfio_add_capabilities()

Below two are integer-valued functions:
vfio_add_vendor_specific_cap()
vfio_setup_pcie_cap()

But the returned integer is only used for check succeed/failure.
Change them all to return bool so now all capability related
functions follow the coding standand in qapi/error.h to return
bool.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>