libxc: Pause & unpause the domain in xc_mem_event_enable based on its initial state.

In an attempt to mitigate XSA-99, xc_mem_event_enable ensures that the domain is paused
for the duration of the event ring setup. However, it disregards the initial state of
the domain, which might already be paused, resulting in 1) an uneccessary hypercall to
pause it again and 2) unpauses it unconditionally which is an opaque and potentially
unwanted side-effect. This patch fixes both issues.

Signed-off-by: Tamas K Lengyel <tamas.k.lengyel@tum.de>