libxl: fd events: Break out fd_occurs

libxl: fd events: Break out fd_occurs

No functional change, only code motion.

Currently, contrary to this function's name, there are two sites where
efd->func() is called so one of them doesn't go through here just yet.
That will be dealt with in the next commit.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Jim Fehlig <jfehlig@suse.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Jim Fehlig <jfehlig@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>