block-file-emu fails in emu SMP

[gut]

Block-file-emu emulates a block device behavior by accessing a file on the host system.

Processors in emu SMP mode are emulated by creating processes, done by calling fork(). But with fork(), file descriptors are not passed to child processes, and thus accesses to the block device from other processors fail.

On Linux hosts, the only ones on which the SMP mode is supported, we should call clone(CLONE_FILES) to create others processors in order to share all file descriptors.