bhyve: TRIM support in AHCI controller

[Wanpeng Qian]

Hi all

I found the output of diskinfo inside VM of AHCI disk as follow, no
TRIM support.
root at smart:~ #  diskinfo -v ada0
ada0
512          # sectorsize
21474836480 # mediasize in bytes (20G)
41943040    # mediasize in sectors
131072      # stripesize
0            # stripeoffset
41610        # Cylinders according to firmware.
16          # Heads according to firmware.
63          # Sectors according to firmware.
BHYVE SATA DISK # Disk descr.
BHYVE-0B98-104F-E9A7 # Disk ident.
No          # TRIM/UNMAP support
0            # Rotation rate in RPM
Not_Zoned    # Zone Mode

Also camcontrol identify ada0 -v show not support of TRIM

Data Set Management (DSM/TRIM) no

I check to pci_ahci.c, TRIM support is decided by block_if.c's candelete flag.
while candelete is decided here

                strlcpy(arg.name, "GEOM::candelete", sizeof(arg.name));
                arg.len = sizeof(arg.value.i);
                if (ioctl(fd, DIOCGATTR, &arg) == 0)
                        candelete = arg.value.i;

Obvious file backend will not have the candelete flag.

If the backend storage support TRIM, I think we should present TRIM
ability to VM.
Any better idea to check whether backend storage support TRIM or not,
so we can enable it.

Thanks.