maru-tablet: fix unexpected shutdown issue

maru-tablet: fix unexpected shutdown issue

If a user uses this device to hard, the emulator dies unexpectedly.
Its reason is to use the virtio queue.
The tablet device accesses the queue that is not prepared
and then releases it.
Fix the issue and refactoring the code.
 - improve the use of the virtio queue
 - remove unused structure & variables
 - pthread mutex to qemu mutex
 - remove unnecessary function declaration
 - clean up logs.

Change-Id: I8adfd2e89c88185547333a45462eceaa1f061c5b
Signed-off-by: Jinhyung Jo <jinhyung.jo@samsung.com>