Flatten the main loop

First of all, vm_can_run is a misnomer, it actually means "no request
pending". Moreover, there is no need to check all pending requests
twice, the first time via the inner loop check and then again when
actually processing the requests. We can simply remove the inner loop
and do the checks directly.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
