libxl_pci: Coding style of do_pci_add

libxl_pci: Coding style of do_pci_add

do_pci_add is going to be asynchronous, so we start by having a single
path out of the function. All `return`s instead set rc and goto out.

While here, some use of `rc' was used to store the return value of
libxc calls, change them to store into `r'. Also, add the value of `r'
in the error message of those calls.

There were an `out' label that was use it seems to skip setting up the
IRQ, the label has been renamed to `out_no_irq'.

No functional changes.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>