qemu-xen: fix sequence of operations in pt_msix_init()

qemu-xen: fix sequence of operations in pt_msix_init()

Checking the return value of mmap() must be done before adjusting the
value, otherwise failure may not be detected.

Closing the file handle, on the other hand, can be done before checking
the return value.

Finally, printing the errno value without knowing whether the previous
function actually failed is bogus (and superfluous since a subsequent
message prints the strerror() representaton anyway).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>