KVM: Calculate available entries in coalesced mmio ring

Instead of checking whether we'll wrap around, calculate how many entries
are available, and check whether we have enough (just one) for the pending
mmio.

By itself, this doesn't change anything, but it paves the way for making
this function lockless.

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

diff --cc virt/kvm/coalesced_mmio.c
Simple merge