KVM: Close minor race in signal handling

KVM: Close minor race in signal handling

We need to check for signals inside the critical section, otherwise a
signal can be sent which we will not notice.  Also move the check
before entry, so that if the signal happens before the first entry,
we exit immediately instead of waiting for something to happen to the
guest.

Signed-off-by: Avi Kivity <avi@qumranet.com>