main-loop: Acquire main_context lock around os_host_main_loop_wait.

When running virt-rescue the serial console hangs from time to time.
Virt-rescue runs an ordinary Linux kernel "appliance", but there is
only a single idle process running inside, so the qemu main loop is
largely idle.  With virt-rescue >= 1.37 you may be able to observe the
hang by doing:

  $ virt-rescue -e ^] --scratch
  ><rescue> while true; do ls -l /usr/bin; done

The hang in virt-rescue can be resolved by pressing a key on the
serial console.

Possibly with the same root cause, we also observed hangs during very
early boot of regular Linux VMs with a serial console.  Those hangs
are extremely rare, but you may be able to observe them by running
this command on baremetal for a sufficiently long time:

  $ while libguestfs-test-tool -t 60 >& /tmp/log ; do echo -n . ; done

(Check in /tmp/log that the failure was caused by a hang during early
boot, and not some other reason)

During investigation of this bug, Paolo Bonzini wrote:

> glib is expecting QEMU to use g_main_context_acquire around accesses to
> GMainContext.  However QEMU is not doing that, instead it is taking its
> own mutex.  So we should add g_main_context_acquire and
> g_main_context_release in the two implementations of
> os_host_main_loop_wait; these should undo the effect of Frediano's
> glib patch.

This patch exactly implements Paolo's suggestion in that paragraph.

This fixes the serial console hang in my testing, across 3 different
physical machines (AMD, Intel Core i7 and Intel Xeon), over many hours
of automated testing.  I wasn't able to reproduce the early boot hangs
(but as noted above, these are extremely rare in any case).

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1435432
Reported-by: Richard W.M. Jones <rjones@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Message-Id: <20170331205133.23906-1-rjones@redhat.com>
[Paolo: this is actually a glib bug: recent glib versions are also
expecting g_main_context_acquire around g_poll---but that is not
documented and probably not even intended].
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
