build: fix virpidfile on mingw

build: fix virpidfile on mingw

Regression introduced in commit b7e5ca4.

Mingw lacks kill(), but we were only using it for a sanity check;
so we can go with one less check.

Also, on OOM error, this function should outright fail rather than
claim that the pid file was successfully read.

* src/util/virpidfile.c (virPidFileReadPathIfAlive): Skip kill
call where unsupported, and report error on OOM.