'!' allowed character in block and partition names

See https://bugzilla.redhat.com/show_bug.cgi?id=1082208

Signed-off-by: Andy Grover <agrover@redhat.com>
Do not set emulate_write_cache based on write_back param for block

Since the kernel's d0c8b259f (in 3.9+), setting emulate_write_cache for
block backstores has not done anything -- the kernel now actually uses
the queue's REQ_FLUSH flag.

No longer set this attribute, but keep the parameter for backwards
compatibility.

Signed-off-by: Andy Grover <agrover@redhat.com>