kvm__emulate_io: Don't call br_read_unlock() twice on IO error

The IO error path in kvm__emulate_io would call br_read_unlock(), then
goto error, which would call br_read_unlock() again.  Refactor the
control flow to have only one exit path and one call to
br_read_unlock().

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>