util: introduce bitmap_try_new

regular bitmap_new simply aborts if the memory allocation fails.
bitmap_try_new returns NULL on failure and allows for proper
error handling.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
block/iscsi: handle failure on malloc of the allocationmap

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>