Tools: New ACRNCTL command "blkrescan"

This patch adds support for new acrnctl command blkrescan.
Purpose: To add a virtio-blk device to guest VM after launch.
Use case:
1. Anticipate number of block devices that will be needed
   and add empty backend file while launching VM.
   e.g: -s 5, virtio-blk,nodisk
2. Use the following acrnctl command to trigger
   a rescan of virtio-blk device by guest VM, to
   revalidate and update the backend file.
   “acrnctl blkrescan VMname slot,newfilepath”

v5 -> v6:
- Removed unsed #define
- Reduced the size of param len of blkrescan command
  from 512 to 256.

v2 -> v5:
- No change

v1 -> v2:
- Renamed the acrnctl command and APIs from diskplug
  to blkrescan.
- Split the patch into two. This corresponds to
  changes in tools.

Tracked-On: #3051
Signed-off-by: Vijay Dhanraj <vijay.dhanraj@intel.com>
Reviewed-by: Yu Wang <yu1.wang@intel.com>

tools/acrn-manager/acrnctl.h

@@ -59,5 +59,6 @@ int pause_vm(const char *vmname);
 int continue_vm(const char *vmname);
 int suspend_vm(const char *vmname);
 int resume_vm(const char *vmname, unsigned reason);
+int blkrescan_vm(const char *vmname, char *devargs);
 
 #endif				/* _ACRNCTL_H_ */