KVM: Retry KVM_CREATE_VM on EINTR

Upstreaming this change from Android (https://android-review.googlesource.com/54211).

On heavily loaded machines with many VM instances we see KVM_CREATE_VM
failing with EINTR on this path:

kvm_dev_ioctl_create_vm -> kvm_create_vm -> kvm_init_mmu_notifier -> mmu_notifier_register ->  do_mmu_notifier_register -> mm_take_all_locks

which checks if any signals have been raised while it was attaining locks
and returns EINTR.  Retrying the system call greatly improves reliability.

Cc: qemu-stable@nongnu.org
Signed-off-by: thomas knych <thomaswk@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
