Add virResetError

Although virConnCopyLastError and virCopyLastError are provided, using them
requires freeing the string memory area with virResetError.  Therefore, add
virResetError.

Also fix mistakes in the Javadoc "missig api in 0.1.0" list:
- virConnCopyLastError is provided
- virFreeError is not missing in 0.1.0 (it is listed as missing in 0.6.1)

Signed-off-by: Mitsuru Kariya <mitsuru.kariya@nttdata.com>